A trapezoid is a quadrilateral with two parallel sides. In geometry, you may be asked to find a missing side of a trapezoid, given the area and height. For example: a trapezoid has an area of 171 cm^2, a side of 10 cm and a height of 18 cm. How long is the missing side? Finding it takes some basic principles of geometry and a splash of algebra.

Substitute the information given in the question to the formula for finding the area of a trapezoid (A=.5height (base1+base2)). 171=(.5)*18(10+base2).

Multiply out .5 by 18: 171= 9(10+base 2).

Use the distributive property to multiple out the equation: 171= 90+9(base 2).

Use algebra to solve the remainder of the problem. Subtract 90 from both sides: 81=9(base2), then divide each side by 9. 9=base 2.