Pythagorean Theorem. Out of the blue?

When I first heard the Pythagorean Formula, I wondered where did Pythagoras get the idea of that relationship? The teacher showed a right triangle and drew squares upon each side. That was so unintuitive to me. It was a deduction after the fact. It didn’t motivate my understanding of the formula.

Square inscribed in a square
Figure 1. Square inscribed in a square

Years later, I stumbled across a construction that made much more sense. Consider a square. Divide a side anywhere into two parts, named a and b. Then from the first a draw a straight line to point a on the adjacent side. Do the same to the third side, the fourth side, then close the inscribed square by drawing a straight line from the fourth side to the original point a.

Figure 1 shows the original square of side a+b and the inscribed square of side c. 

Area of original square = Area of inscribed square + 4* identical right triangles

(a + b)2 = c2 + 4(½ ab)

a2 + 2ab + b2 =  c2 + 2 ab

a2 + b2 = c2

Perhaps some teachers use the inscribed square to explain the Pythagorean Theorem. I wish mine had.


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