Geometry problem #2

Geometry Level 3

In the image below, B C E D BCED is a square and A C G F ACGF is a parallelogram. B D G BDG and A C B ACB are straight lines. If B E = 8 2 BE=8\sqrt 2 , find the area of A F H \triangle AFH .

25 36 32 64

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1 solution

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 \triangle AFH equals half the area of the square B C E D BCED . But, [ B C E D ] = 1 2 B E 2 \left[ BCED \right]=\dfrac{1}{2}B{{E}^{2}} . Hence, [ A F H ] = 1 2 [ B C E D ] = 1 2 × 1 2 × ( 8 2 ) 2 = 32 \left[ AFH \right]=\frac{1}{2}\left[ BCED \right]=\frac{1}{2}\times \frac{1}{2}\times {{\left( 8\sqrt{2} \right)}^{2}}=\boxed{32}

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