A neat way to calculate the area of a rectangle

I was sitting down, solving one of Catriona Shearer‘s geometry puzzles when I found this (by myself):

Look, I know it’s not the best of drawings, but it’s what I did.

If we call the long diagonal x, and the line segment perpendicular to the diagonal y, then the area of the triangle = xy/2, and the rectangle is made up of two of those triangles with area xy/2, so the total area of the rectangle = (xy/2) * 2 = xy.

I guess there could be another way to prove it. But this is the neat way I found!


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