Area of Triangles & Trapezoids for Kids

By Kristen May
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One of the main categories of math that elementary and middle school students need to learn is how to find the area of a shape. The easiest shapes are squares and rectangles, but triangles and trapezoids are right behind them. Teach kids the formulas for area and equip them in how to find the numbers to plug into the formulas.

Triangle Formula

Calculate the area of a triangle by multiplying the length of the base by the triangle's height and dividing the result by 2. The formula is usually written as 1/2bh. The base is the flat side that usually runs along the bottom of the triangle. The height is the shortest distance from the point of the triangle opposite the base down to the base. The height is always measured in a direction perpendicular to the base.

Triangle Example

Say you have a right triangle and you measure the two edges forming the right angle to find that they are 3 inches and 4 inches long. If you look at the triangle so the side that is 4 inches long is running horizontally, the side that is 3 inches long will be vertical. Therefore, the base is 4 inches and the height is 3 inches. Calculate the area by multiplying 1/2 by 4 by 3. The solution is that the area is 6 square inches.

Trapezoid Formula

The area of a trapezoid is the sum of the bases times the height, all divided by 2. The way to write the formula is 1/2 * (b1 + b2)* h. The bases on the trapezoid are the two parallel edges. In all trapezoids, by definition, two of the four edges run parallel to one another. The top edge is called b1 and the bottom is b2. The height of the trapezoid is the shortest distance between b1 and b2, which makes a right angle with both bases.

Trapezoid Example

Say you have a trapezoid with two parallel edges that are 8 inches long and 5 inches long. The distance between these parallel edges is 3 inches. To calculate the area, start by adding together the lengths of the bases, 8 inches and 5 inches, to get 13 inches. Multiply this by the height of 5 inches to get 65 square inches. Divide by 2 to find the area of the trapezoid, which is 32.5 square inches.