[abousoun]

It's very simple ... just look carefully at the first picture, you'll notice that the "hypotenuse" is not a straight line, it's slightly concave rather straight ...
once you draw a straight line as in the second picture or maybe it's a slightly convexed line, this line will fill the slight concavity in the first picture ... then when you calculate the surface of that area it will be equal to a small square that you found in the next picture ...

Thank You ...