A trapezoid is a foursided (quadrilateral) shape that includes two parallel sides called bases. An equation is needed to calculate the various dimensions of a trapezoid. The base equation is as follows: area = ((b1 + b2) / 2) a. In this equation, b = base, and a = altitude (height). In simple translation, the area is equal to the average length of the bases, multiplied by the altitude. To find the altitude, the inverse of this equation must be used: a = area / ((b1 + b2) / 2), dividing the area by the average length of the bases.
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Altitude

Write this example to see how it works; then simply replace the numbers here with your own numbers: a = 128 / ((12 + 20) / 2. This is the inverted formula for the altitude, itself.

Add the two bases together with a calculator. In this case, the bases are 12 and 20. The sum of these is 32.

Divide 32 by 2, according to the formula, to get 16.

Divide the area, 128, by 16. The answer is 8. Since 8 = 128 / ((12 + 20) / 2, that means 8 is the altitude of this trapezoid.