The triangle formed by the boxes is a right angle triangle. Recall,
Area of square = length^2
The length of the sides of the given traingle are
a = opposite side
b = adjacent side
c = hypotenuse
The pythagorean theorem is expressed as
Hypotenuse^2 = opposite side^2 + adjacent side^2
This means that
c^2 = a^2 + b^2
The length of each side of the smallest square is a. Thus, it's area is a^2
The length of each side of the medium square is b. Thus, it's area is b^2
The length of each side of the largest square is c. Thus, it's area is c^2
Thus, in proving the pythagorean theorem,
when the sum of the area of the small and medium squares equal the area of the largest square., then the pythagorean theorem is proven