AC2 + BD2 = 4AB2 Solution: The diagonals of a rhombus bisect each other at right angles. $\Rightarrow\text{OA}=\frac{1}{2}\text{AC}$ and $\text{OB}=\frac{1}{2}\text{BD}$ Also, $\angle\text{AOB}=90^{\circ}$ In right $\triangle\text{AOB},$ $\therefore\text{AB}^2=\text{OA}^2+\text{OB}^2$ ...(By Pythagoras theorem) $\Rightarrow\text{AB}^2=\frac{1}{4}\text{AC}^2+\frac{1}{4}\text{BD}^2$ $\Rightarrow\text{AC}^2+\text{BD}^2=4\text{AB}^2$
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