In the image below, is a square and is a parallelogram. and are straight lines. If , find the area of .
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In the animation, every moving triangle preserves its area, because the varying vertex moves parallel to the opposite highlighted base. Hence, the area of △ A F H equals half the area of the square B C E D . But, [ B C E D ] = 2 1 B E 2 . Hence, [ A F H ] = 2 1 [ B C E D ] = 2 1 × 2 1 × ( 8 2 ) 2 = 3 2