## Elementary Geometry for College Students (6th Edition)

We place the parallelogram at the origin. We call its coordinates: $A: 0,0 \\ B: b,c \\ C: a+b, c \\ D: a, 0$ Thus, we compare the lengths of opposite sides: $l_1= a$ $l_2=a+b-b =a$ $l_3 = \sqrt{c^2 + b^2}$ $l_4 = \sqrt{b^2 +c^2}$ We see that the lengths of opposite sides are equal.