[HairOfTheDog]

There is an optical illusion that would suggest an arc, but there is no arc. I confirm with Photoshop that the hypotenuse is a straight line in both.

[ecurbh]

OK, there is something hinky going on here...

Area = Base x Height / 2

Overall area of both shapes should be 32.5 (13 x 5 / 2)

Area of the red triangle is 12 (8 x 3 / 2)

Area of the green triangle is 5 (5 x 2 / 2)

Area of the other shapes are 7 and 8...

But 12 + 5 + 7 + 8 = 32!

That missing .5 is the key to this. But that's where I get stuck. My brother is a mathematics professor, I'll have to email this to him.