## Elementary Geometry for College Students (7th Edition)

We consider a rectangle. where the sides are a, b, c, and d and where the diagonals are A and B. Thus, we obtain: $A^2 = a^2 + b^2$ And: $B^2 = c^2 + d^2$ Adding these gives: $A^2 + B^2 = a^2 + b^2 +c^2 + d^2$ Thus, the proof is completed.