The many-sided trapezoid ... It can be arbitrary, isosceles or rectangular. And in each case, you need to know how to find the area of the trapezoid. Of course, the basic formulas are the easiest to remember. But sometimes it is easier to use the one that is derived taking into account all the features of a particular geometric figure.
A few words about the trapezoid and its elements
Any quadrangle with two sides parallel can be called a trapezoid. V general case they are not equal and are called bases. The larger one is the bottom one and the other is the top one.
The other two sides are sideways. For an arbitrary trapezoid, they have different lengths. If they are equal, then the figure becomes isosceles.
If suddenly the angle between any side and the base turns out to be equal to 90 degrees, then the trapezoid is rectangular.
All these features can help in solving the problem of how to find the area of a trapezoid.
Among the elements of the figure that may be indispensable in solving problems, we can single out the following:
- height, that is, a segment perpendicular to both bases;
- the middle line, which has at its ends the midpoints of the lateral sides.
What is the formula to calculate the area if the bases and height are known?
This expression is given as the main one, because most often you can find out these values, even when they are not given explicitly. So, to understand how to find the area of a trapezoid, you need to add both bases and divide them in two. Then multiply the resulting value by the height value.
If we designate the bases with the letters a 1 and a 2, the height - n, then the formula for the area will look like this:
S = ((a 1 + a 2) / 2) * n.
The formula by which the area is calculated if its height and center line are given
If you look closely at the previous formula, you will easily notice that there is clearly a midline value in it. Namely, the sum of the bases divided by two. Let the middle line be denoted by the letter l, then the formula for the area will be like this:
S = l * n.
The ability to find the area by diagonals
This method will help if you know the angle formed by them. Suppose that the diagonals are denoted by the letters d 1 and d 2, and the angles between them are α and β. Then the formula for how to find the area of a trapezoid will be written as follows:
S = ((q 1 * q 2) / 2) * sin α.
In this expression, you can easily replace α with β. The result will not change.
How to find out the area if all sides of the figure are known?
There are also situations when the sides are known in this figure. This formula is cumbersome and difficult to remember. But probably. Let the sides have the designation: at 1 and at 2, the base of a 1 is greater than a 2. Then the area formula will look like this:
S = ((a 1 + a 2) / 2) * √ (in 1 2 - [(a 1 - a 2) 2 + in 1 2 - in 2 2) / (2 * (a 1 - a 2)) ] 2).
Methods for calculating the area of an isosceles trapezoid
The first one is connected with the fact that a circle can be inscribed in it. And, knowing its radius (it is denoted by the letter r), as well as the angle at the base - γ, you can use the following formula:
S = (4 * r 2) / sin γ.
The last general formula, which is based on the knowledge of all sides of the figure, will be greatly simplified due to the fact that the sides have the same meaning:
S = ((a 1 + a 2) / 2) * √ (b 2 - [(a 1 - a 2) 2 / (2 * (a 1 - a 2))] 2).
Methods for calculating the area of a rectangular trapezoid
It is clear that any of the above will be suitable for an arbitrary figure. But sometimes it is useful to know about one feature of such a trapezoid. It consists in the fact that the difference between the squares of the lengths of the diagonals is equal to the difference made up of the squares of the bases.
Often the formulas for the trapezoid are forgotten, while the expressions for the areas of the rectangle and triangle are remembered. Then a simple way can be applied. Divide the trapezoid into two shapes if it is rectangular, or three. One will definitely be a rectangle, and the second, or the other two, will be triangles. After calculating the areas of these figures, all that remains is to add them.
This is a fairly simple way to find the area of a rectangular trapezoid.
What if the coordinates of the vertices of the trapezoid are known?
In this case, you need to use an expression that allows you to determine the distance between points. It can be applied three times: to find out both bases and one height. And then just apply the first formula, which is described a little above.
To illustrate such a method, the following example can be given. Vertices with coordinates A (5; 7), B (8; 7), C (10; 1), D (1; 1) are given. You need to find out the area of the figure.
Before finding the area of the trapezoid, you need to calculate the lengths of the bases from the coordinates. You will need the following formula:
segment length = √ ((difference of the first coordinates of points) 2 + (difference of second coordinates of points) 2).
The upper base is designated AB, which means that its length will be equal to √ ((8-5) 2 + (7-7) 2) = √9 = 3. Lower - SD = √ ((10-1) 2 + (1-1 ) 2) = √81 = 9.
Now we need to draw the height from the top to the bottom. Let its beginning be at point A. The end of the segment will be on the lower base at the point with coordinates (5; 1), let it be point H. The length of the segment AH will be equal to √ ((5-5) 2 + (7-1) 2 ) = √36 = 6.
It remains only to substitute the resulting values into the formula for the area of the trapezoid:
S = ((3 + 9) / 2) * 6 = 36.
The problem was solved without units of measurement, because the scale of the coordinate grid was not specified. It can be either a millimeter or a meter.
Examples of tasks
No. 1. Condition. The angle between the diagonals of an arbitrary trapezoid is known, it is equal to 30 degrees. The smaller diagonal has a value of 3 dm, and the second is 2 times larger than it. It is necessary to calculate the area of the trapezoid.
Solution. First you need to find out the length of the second diagonal, because without this it will not be possible to count the answer. It is not difficult to calculate it, 3 * 2 = 6 (dm).
Now we need to use a suitable formula for the area:
S = ((3 * 6) / 2) * sin 30º = 18/2 * ½ = 4.5 (dm 2). The problem has been solved.
Answer: the area of the trapezoid is 4.5 dm 2.
No. 2. Condition. In the trapezoid of AVSD, the bases are the segments of blood pressure and BC. Point E is the middle of the SD side. From it, a perpendicular is drawn to line AB, the end of this segment is designated by the letter N. It is known that the lengths AB and EH are 5 and 4 cm, respectively. It is necessary to calculate the area of the trapezoid.
Solution. First you need to make a drawing. Since the value of the perpendicular is less than the side to which it is drawn, the trapezoid will be slightly elongated upward. So EH will be inside the figure.
To clearly see the progress of solving the problem, you will need to perform additional construction. Namely, draw a straight line that will be parallel to the AB side. The intersection points of this straight line with HELL are P, and with the continuation of BC - X. The resulting figure ВХРА is a parallelogram. Moreover, its area is equal to the required one. This is due to the fact that the triangles obtained with the additional construction are equal. This follows from the equality of the side and the two angles adjacent to it, one is vertical, the other is criss-cross.
You can find the area of a parallelogram using a formula that contains the product of the side and the height dropped on it.
Thus, the area of the trapezoid is 5 * 4 = 20 cm 2.
Answer: S = 20 cm 2.
No. 3. Condition. Elements of an isosceles trapezoid have the following meanings: lower base - 14 cm, upper - 4 cm, sharp corner- 45º. You need to calculate its area.
Solution. Let the smaller base be designated BC. The height drawn from point B will be called BH. Since the angle is 45º, the triangle ABN will turn out to be rectangular and isosceles. Hence, AH = BH. And NA is very easy to find. It is equal to half the difference in bases. That is (14 - 4) / 2 = 10/2 = 5 (cm).
The bases are known, the height is calculated. You can use the first formula, which was considered here for an arbitrary trapezoid.
S = ((14 + 4) / 2) * 5 = 18/2 * 5 = 9 * 5 = 45 (cm 2).
Answer: The required area is 45 cm 2.
No. 4. Condition. There is an arbitrary trapezoid AVSD. On its lateral sides, points O and E are taken, so that OE is parallel to the base of blood pressure. The area of the AOED trapezium is five times larger than that of the CFE. Calculate the OE value if the base lengths are known.
Solution. You will need to draw two parallel AB straight lines: the first through point C, its intersection with OE - point T; the second through E and the point of intersection with blood pressure will be M.
Let the unknown OE = x. The height of the smaller trapezoid OVSE - n 1, the greater AOED - n 2.
Since the areas of these two trapezoids are related as 1 to 5, we can write the following equality:
(x + a 2) * n 1 = 1/5 (x + a 1) * n 2
n 1 / n 2 = (x + a 1) / (5 (x + a 2)).
The heights and sides of the triangles are proportional in construction. Therefore, one more equality can be written:
n 1 / n 2 = (x - a 2) / (a 1 - x).
In two recent entries on the left side there are equal values, which means that we can write that (x + a 1) / (5 (x + a 2)) is equal to (x - a 2) / (a 1 - x).
A number of transformations are required here. First multiply crosswise. Brackets will appear that indicate the difference of the squares, after applying this formula, you get a short equation.
In it, you need to open the brackets and transfer all terms from the unknown "x" to left side, and then extract the square root.
Answer: x = √ ((a 1 2 + 5 a 2 2) / 6).
(S) of a trapezoid, start calculating the height (h) by finding the half-sum of the lengths of the parallel sides: (a + b) / 2. Then divide the area by the resulting value - the result will be the desired value: h = S / ((a + b) / 2) = 2 * S / (a + b).
Knowing the length of the center line (m) and the area (S), you can simplify the formula from the previous step. By definition, the middle line of a trapezoid is equal to the half-sum of its bases, so to calculate the height (h) of a figure, simply divide the area by the length of the middle line: h = S / m.
It is possible to determine the height (h) of such and in the event that only the length of one of the lateral sides (c) and the angle (α) formed by it and the long base are given. In this case, one should consider, formed by this side, the height and a short segment of the base, which is cut off by the height lowered to it. This triangle will be rectangular, the known side will be the hypotenuse, and the height will be the leg. The ratio of the lengths to the hypotenuse is equal to the angle opposite the leg, so to calculate the height of the trapezoid, multiply the known side length by the sine of the known angle: h = c * sin (α).
The same triangle should be considered if the length of the lateral side (c) and the value of the angle (β) between it and the other (short) base are given. In this case, the angle between the lateral side (hypotenuse) and the height (leg) will be 90 ° less than the angle known from the conditions: β-90 °. Since the ratio of the lengths of the leg and the hypotenuse is equal to the cosine of the angle between them, calculate the height of the trapezoid by multiplying the cosine of the angle reduced by 90 ° by the length of the side side: h = c * cos (β-90 °).
If a circle of known radius (r) is inscribed, calculating the height (h) will be very simple and will not require any other parameters. Such a circle, by definition, must each of the bases have only one point and these points will lie on the same line with the center. This means that the distance between them will be equal to the diameter (twice the radius) drawn perpendicular to the bases, that is, coinciding with the height of the trapezoid: h = 2 * r.
A trapezoid is a quadrilateral in which two sides are parallel and the other two are not. The height of a trapezoid is a segment drawn perpendicularly between two parallel lines. It can be calculated in different ways depending on the initial data.
You will need
- Knowledge of the sides, bases, centerline of the trapezoid, as well as, optionally, its area and / or perimeter.
Instructions
Let's say there is a trapezoid with the same data as in Figure 1. Draw 2 heights, we get, which has 2 smaller sides with legs of right-angled triangles. Let's denote the smaller roll as x. He is in
Trapezoid is called a quadrangle for which only two the sides are parallel to each other.
They are called the bases of the figure, the rest are called the sides. A parallelogram is considered a special case of a figure. There is also a curved trapezoid that includes a function graph. The formulas for the area of a trapezoid include almost all of its elements, and the best solution is selected depending on the specified values.
The main roles in the trapezoid are assigned to the height and midline. middle line Is the line connecting the midpoints of the sides. Height the trapezoid is held at right angles from the top corner to the base.
The area of the trapezoid through the height is equal to the product of the half-sum of the lengths of the bases, multiplied by the height:
If, according to the conditions, the middle line is known, then this formula is greatly simplified, since it is equal to the half-sum of the lengths of the bases:
If, according to the conditions, the lengths of all sides are given, then we can consider an example of calculating the area of a trapezoid through this data: 
Suppose a trapezoid is given with bases a = 3 cm, b = 7 cm and lateral sides c = 5 cm, d = 4 cm. We find the area of the figure: 
Area of an isosceles trapezoid
An isosceles or, as it is also called, an isosceles trapezoid is considered a separate case.
Finding the area of an isosceles (isosceles) trapezoid is also a special case. The formula is displayed in various ways - through the diagonals, through the corners adjacent to the base and the radius of the inscribed circle.
If, according to the conditions, the length of the diagonals is specified and the angle between them is known, you can use the following formula:
Remember that the diagonals of an isosceles trapezoid are equal!
That is, knowing one of their bases, the side and the angle, you can easily calculate the area.
Curved trapezoid area
A separate case is curved trapezoid... It is located on the coordinate axis and is limited to the graph of a continuous positive function.
Its base is located on the X-axis and is limited by two points: 
Integrals help you calculate the area of a curved trapezoid.
The formula is written like this:
Consider an example of calculating the area of a curved trapezoid. The formula requires certain knowledge to work with definite integrals. First, let's look at the value of a definite integral: 
Here F (a) is the value of the antiderivative function f (x) at point a, F (b) is the value of the same function f (x) at point b. 
Now let's solve the problem. The figure shows a curved trapezoid limited by a function. Function 
We need to find the area of the selected figure, which is a curvilinear trapezoid bounded from above by a graph, to the right by a straight line x = (- 8), to the left by a straight line x = (- 10) and the OX axis from below.
We will calculate the area of this figure by the formula: 
A function is given to us by the conditions of the problem. Using it, we will find the values of the antiderivative at each of our points:
Now
Answer: the area of a given curved trapezoid is 4.
There is nothing difficult in calculating this value. Only the utmost care in the calculations is important.
The practice of last year's USE and GIA shows that geometry problems cause difficulties for many schoolchildren. You can easily cope with them if you memorize all the necessary formulas and practice solving problems.
In this article, you will see formulas for finding the area of a trapezoid, as well as examples of problems with solutions. You can find the same ones in KIMs at certification exams or at olympiads. Therefore, treat them carefully.
What you need to know about a trapezoid?
First, let's remember that trapezoid called a quadrangle, which has two opposite sides, they are also called bases, are parallel, and the other two are not.
The height can also be lowered in the trapezoid (perpendicular to the base). The middle line is drawn - this is a straight line that is parallel to the bases and is equal to half of their sum. And also diagonals, which can intersect, forming acute and obtuse corners. Or in individual cases, at right angles. In addition, if the trapezoid is isosceles, a circle can be inscribed into it. And describe a circle around it.
Area formulas for a trapezoid
To begin with, consider the standard formulas for finding the area of a trapezoid. We will consider ways to calculate the area of an isosceles and curved trapeziums below.
So, imagine that you have a trapezoid with bases a and b, in which the height h is lowered to the larger base. Calculating the area of the figure in this case is as easy as shelling pears. You just need to divide by two the sum of the lengths of the bases and multiply what you get by the height: S = 1/2 (a + b) * h.
Let us take another case: suppose, in the trapezoid, in addition to the height, the middle line m is drawn. We know the formula for finding the length of the midline: m = 1/2 (a + b). Therefore, we can rightfully simplify the formula for the area of a trapezoid to of the following type: S = m * h... In other words, to find the area of a trapezoid, you must multiply the midline by the height.
Consider another option: in the trapezoid, diagonals d 1 and d 2 are drawn, which do not intersect at a right angle α. To calculate the area of such a trapezoid, you need to divide by two the product of the diagonals and multiply the result by the sin of the angle between them: S = 1 / 2d 1 d 2 * sinα.
Now consider the formula for finding the area of a trapezoid if nothing is known about it, except for the lengths of all its sides: a, b, c and d. This is a cumbersome and complex formula, but it will be useful for you to remember it, just in case: S = 1/2 (a + b) * √c 2 - ((1/2 (b - a)) * ((b - a) 2 + c 2 - d 2)) 2.
By the way, the above examples are also true for the case when you need the area formula rectangular trapezoid... This is a trapezoid, the side of which is adjacent to the bases at right angles.
Isosceles trapezoid
A trapezoid, the sides of which are equal, is called isosceles. We will consider several options for the formula for the area of an isosceles trapezoid.
The first option: for the case when a circle with a radius r is inscribed inside the isosceles trapezoid, and the lateral side and the larger base form an acute angle α. A circle can be inscribed in a trapezoid, provided that the sum of the lengths of its bases is equal to the sum of the lengths of the sides.
The area of an isosceles trapezoid is calculated as follows: multiply the square of the radius of the inscribed circle by four and divide it all by sinα: S = 4r 2 / sinα... Another area formula is a special case for the case when the angle between the large base and the side is 30 0: S = 8r 2.
The second option: this time we take an isosceles trapezoid, in which, in addition, the diagonals d 1 and d 2 are drawn, as well as the height h. If the diagonals of the trapezoid are mutually perpendicular, the height is half the sum of the bases: h = 1/2 (a + b). Knowing this, it is easy to transform the already familiar formula for the area of a trapezoid into the following form: S = h 2.
Formula for the area of a curved trapezoid
Let's start by looking at what a curved trapezoid is. Imagine a coordinate axis and a graph of a continuous and non-negative function f that does not change sign within a given segment on the x-axis. The curvilinear trapezoid is formed by the graph of the function y = f (x) - at the top, the x-axis - at the bottom (segment), and on the sides - the lines drawn between points a and b and the graph of the function.
Calculate the area of such irregular shape cannot be done in the above ways. Here you need to apply mathematical analysis and use the integral. Namely: the Newton-Leibniz formula - S = ∫ b a f (x) dx = F (x) │ b a = F (b) - F (a)... In this formula, F is the antiderivative of our function on the selected segment. And the area of the curvilinear trapezoid corresponds to the increment of the antiderivative on a given segment.
Examples of tasks
To make all these formulas settle in your head better, here are some examples of problems for finding the area of a trapezoid. It will be best if you first try to solve the problems yourself, and only then check the answer received with the ready-made solution.
Task number 1: Given a trapezoid. Its larger base is 11 cm, the smaller one is 4 cm. Diagonals are drawn in the trapezoid, one 12 cm long, the other 9 cm long.
Solution: Construct a trapezoid AMRS. Draw line PX through vertex P so that it turns out to be parallel to the MC diagonal and intersects line AC at point X. You will get a triangle ARX.
We will consider two figures obtained as a result of these manipulations: the ARX triangle and the CMRX parallelogram.
Thanks to the parallelogram, we learn that PX = MC = 12 cm and CX = MR = 4cm. Where can we calculate the side AX of triangle ARX: AX = AC + CX = 11 + 4 = 15 cm.
We can also prove that the triangle ARX is rectangular (for this, apply the Pythagorean theorem - AX 2 = AR 2 + PX 2). And calculate its area: S APX = 1/2 (AP * PX) = 1/2 (9 * 12) = 54 cm 2.
Next, you need to prove that triangles AMP and PCX are equal. The basis will be the equality of the sides МР and СХ (already proved above). And also the heights that you lower on these sides - they are equal to the height of the AMRS trapezoid.
All this will allow you to assert that S AMPC = S APX = 54 cm 2.
Task number 2: Given a trapezoid KRMS. Points O and E are located on its lateral sides, while OE and KC are parallel. It is also known that the areas of the trapeziums ORME and OCE are in a ratio of 1: 5. PM = a and KC = b. It is required to find OE.
Solution: Draw a straight line through point M, parallel to the RC, and designate the point of its intersection with OE by T. A - the point of intersection of a straight line drawn through point E parallel to the RC, with the base of the COP.
Let us introduce one more notation - OE = x. And also the height h 1 for the TME triangle and the height h 2 for the AEC triangle (you can independently prove the similarity of these triangles).
We will assume that b> a. The areas of the trapeziums ORME and OKSE are related as 1: 5, which gives us the right to draw up the following equation: (x + a) * h 1 = 1/5 (b + x) * h 2. Let's transform and get: h 1 / h 2 = 1/5 * ((b + x) / (x + a)).
Since triangles TME and AEC are similar, we have h 1 / h 2 = (x - a) / (b - x). Combine both records and get: (x - a) / (b - x) = 1/5 * ((b + x) / (x + a)) ↔ 5 (x - a) (x + a) = (b + x) (b - x) ↔ 5 (x 2 - a 2) = (b 2 - x 2) ↔ 6x 2 = b 2 + 5a 2 ↔ x = √ (5a 2 + b 2) / 6.
Thus, OE = x = √ (5a 2 + b 2) / 6.
Conclusion
Geometry is not the easiest science, but you can surely be able to handle examination tasks... It is enough to show a little perseverance in preparation. And, of course, remember all the necessary formulas.
We tried to collect in one place all the formulas for calculating the area of a trapezoid so that you can use them when you prepare for exams and review the material.
Be sure to share this article with your classmates and friends in social networks... Let there be more good grades for the Unified State Exam and the State Examination Agency!
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In order to feel confident in geometry lessons and successfully solve problems, it is not enough to learn formulas. First of all, you need to understand them. To be afraid, let alone hate formulas, is unproductive. In this article accessible language will be analyzed different ways search for the area of the trapezoid. For a better understanding of the corresponding rules and theorems, we will pay some attention to its properties. This will help you understand how the rules work and when you should apply certain formulas.
Defining a trapezoid
What is this figure in general? A trapezoid is a polygon of four corners with two parallel sides. The other two sides of the trapezoid can be tilted at different angles. Its parallel sides are called bases, and for non-parallel sides the name "sides" or "thighs" is used. Such figures are quite common in everyday life... The contours of the trapezoid can be seen in the silhouettes of clothing, interior items, furniture, dishes and many others. The trapezoid happens different types: versatile, isosceles and rectangular. We will analyze their types and properties in more detail later in the article.
Trapezoid properties
Let us dwell briefly on the properties of this figure. The sum of the angles adjacent to either side always equals 180 °. It should be noted that all the angles of the trapezoid add up to 360 °. The trapezoid has the concept of a midline. If you connect the midpoints of the sides with a segment, this will be the middle line. It is designated by m. The middle line has important properties: it is always parallel to the bases (we remember that the bases are also parallel to each other) and is equal to their half-sum:
This definition must be learned and understood, because it is the key to solving many problems!
At the trapezoid, you can always lower the height to the base. Height is a perpendicular, often denoted by the symbol h, that is drawn from any point on one base to another base or its extension. The midline and height will help you find the area of the trapezoid. Such tasks are the most common in school course geometry and regularly appear among the control and examination papers.
The simplest formulas for the area of a trapezoid
Let's analyze the two most popular and simple formulas, with the help of which the area of the trapezoid is found. It is enough to multiply the height by half the sum of the bases to easily find what you are looking for:
S = h * (a + b) / 2.
In this formula, a, b denote the base of the trapezoid, h - the height. For ease of perception, in this article, the multiplication signs are marked with a (*) symbol in the formulas, although the multiplication sign is usually omitted in the official reference books.
Let's look at an example.
Given: a trapezoid with two bases equal to 10 and 14 cm, the height is 7 cm. What is the area of the trapezoid?
Let's analyze the solution to this problem. Using this formula, you first need to find the half-sum of the bases: (10 + 14) / 2 = 12. So, the half-sum is 12 cm. Now we multiply the half-sum by the height: 12 * 7 = 84. The sought-after is found. Answer: the area of the trapezoid is 84 sq. cm.
The second famous formula reads: the area of the trapezoid is equal to the product of the midline and the height of the trapezoid. That is, in fact, it follows from the previous concept of the middle line: S = m * h.
Using diagonals for calculations
Another way to find the area of a trapezoid is actually not that difficult. It is associated with its diagonals. According to this formula, to find the area, you need to multiply the half-product of its diagonals (d 1 d 2) by the sine of the angle between them:
S = ½ d 1 d 2 sin a.
Consider a problem that shows the application of this method. Given: a trapezoid with a diagonal length of 8 and 13 cm, respectively. The angle a between the diagonals is 30 °. Find the area of the trapezoid.
Solution. Using the above formula, it is easy to calculate what is required. As you know, sin 30 ° is 0.5. Therefore, S = 8 * 13 * 0.5 = 52. Answer: the area is 52 sq. cm.
We are looking for the area of an isosceles trapezoid
The trapezium can be isosceles (isosceles). Its sides are the same AND the angles at the bases are equal, which is well illustrated in the figure. An isosceles trapezoid has the same properties as a regular trapezoid, plus a number of special ones. A circle can be described around an isosceles trapezoid, and a circle can be inscribed in it.
What are the methods for calculating the area of such a figure? The method below will require a lot of computation. To use it, you need to know the values of the sine (sin) and cosine (cos) of the angle at the base of the trapezoid. To calculate them, either Bradis tables or an engineering calculator are required. Here's the formula:
S = c* sin a*(a - c* cos a),
where with- lateral thigh, a- angle at the bottom base.
An isosceles trapezoid has diagonals equal length... The converse is also true: if a trapezoid has equal diagonals, then it is isosceles. Hence the following formula, which helps to find the area of a trapezoid, is the half-product of the square of the diagonals by the sine of the angle between them: S = ½ d 2 sin a.
Find the area of a rectangular trapezoid
A special case of a rectangular trapezoid is known. This is a trapezoid in which one lateral side (its thigh) adjoins the bases at right angles. It has the properties of an ordinary trapezoid. In addition, she has a very interesting feature... The difference between the squares of the diagonals of such a trapezoid is equal to the difference between the squares of its bases. For it, all the previously given methods for calculating the area are used.
Applying ingenuity
There is one trick that can help in case of forgetfulness of specific formulas. Let's take a closer look at what a trapezoid is. If we mentally divide it into parts, then we get familiar and understandable geometric shapes: a square or rectangle and a triangle (one or two). If you know the height and sides of the trapezoid, you can use the formulas for the area of a triangle and a rectangle, and then add all the resulting values.
Let us illustrate this with the following example. You are given a rectangular trapezoid. Angle C = 45 °, angles A, D are 90 °. The upper base of the trapezoid is 20 cm, the height is 16 cm. It is required to calculate the area of the figure.
This figure obviously consists of a rectangle (if the two angles are 90 °) and a triangle. Since the trapezoid is rectangular, therefore, its height is equal to its lateral side, that is, 16 cm. We have a rectangle with sides of 20 and 16 cm, respectively. Consider now a triangle whose angle is 45 °. We know that one side of it is 16 cm. Since this side is at the same time the height of the trapezoid (and we know that the height drops to the base at a right angle), therefore, the second angle of the triangle is 90 °. Hence the remaining angle of the triangle is 45 °. As a result, we get a rectangular isosceles triangle whose two sides are the same. This means that the other side of the triangle is equal to the height, that is, 16 cm. It remains to calculate the area of the triangle and rectangle and add the resulting values.
The area of a right-angled triangle is equal to half the product of its legs: S = (16 * 16) / 2 = 128. The area of a rectangle is equal to the product of its width and length: S = 20 * 16 = 320. We found the required: the area of the trapezoid S = 128 + 320 = 448 sq. see. You can easily double-check yourself using the above formulas, the answer will be identical.
Using Pick's formula
Finally, we present one more original formula that helps to find the area of a trapezoid. It is called Pick's formula. It is convenient to use it when the trapezoid is drawn on checkered paper. Similar tasks are often found in the materials of the GIA. It looks like this:
S = M / 2 + N - 1,
in this formula M is the number of nodes, i.e. the intersections of the lines of the figure with the lines of the cells on the borders of the trapezoid (orange points in the figure), N is the number of nodes inside the figure (blue points). It is most convenient to use it when finding the area of an irregular polygon. Nevertheless, the larger the arsenal of techniques used, the fewer errors and the better the results.
Of course, the information given does not exhaust the types and properties of the trapezoid, as well as the methods for finding its area. This article provides an overview of its most important characteristics. In solving geometric problems, it is important to act gradually, to start with easy formulas and problems, to consistently consolidate understanding, to move to another level of complexity.
Putting together the most common formulas will help students navigate in various ways to calculate the area of a trapezoid and better prepare for tests and control works on this topic.
