Let $ABCD$ be a parallelogram with side lengths $u,v$ and whose diagonals have lengths $d_1$ and $d_2$ then $$2u^2+2v^2=d_1^2 + d_2^2.$$

<45>> \begin{pspicture*}(-1,-1)(6,3) \pspolygon(0,0)(4,0)(5,2)(1,2) \uput[270](2,-0.3){$u$} \uput[180](0.3,1){$v$} \qline(0,0)(5,2) \qline(4,0)(1,2) \uput[0](1.7,0.5){$d_1$} \uput[0](1.8,1.5){$d_2$} \end{pspicture*} } \end{center} \end{document}