How Can I Find The Unknown Base Of A Trapezoid?

The equation for area of a trapezoid is a=1/2*h(b₁+b₂).  To find the measure of missing base, you will multiply 1/2 times h, then divide each side by that product.  That will cancel the 1/2*h on one side, leaving you with area over half the height equal to b₁ + b₂.  Then subtract b₁ from each side, which will result in the following equation: (area / (1/2*h)) - b₁ = b₂.
 
To plug in numbers, let's say area = 28; height = 4; and b₁ =8.  We need to find b₂.  When we substitute in to the original equation, it looks like this:
28 = 1/2*(4)(8 + b₂)   Substitution
28/2 = (8 + b₂)          Divide both sides by 1/2 height
14 - 8 = b₂                 Subtract b₁ from both sides
6 = b₂                        Simplify
The measure of the missing base is 6.

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