Prove that the sum of the squares of the sides of a rhombus is equal to the sum of the squares of its diagonals.
To do:
We have to prove that the sum of the squares of the sides of a rhombus is equal to the sum of the squares of its diagonals.
Solution:
Let $ABCD$ be a rhombus in which diagonals $AC$ and $BD$ intersect each other at $O$.
We know that,
The two diagonals of a rhombus are perpendicular.
Therefore,
In $\triangle \mathrm{AOB}$,
$\mathrm{AB}^{2}=\mathrm{AO}^{2}+\mathrm{OB}^{2}$
$\mathrm{AB}^{2}=(\frac{1}{2} \mathrm{AC})^{2}+(\frac{1}{2} \mathrm{BD})^{2}$
$\mathrm{AB}^{2}=\frac{1}{4} \mathrm{AC}^{2}+\frac{1}{4} \mathrm{BD}^{2}$
$4 \mathrm{AB}^{2}=\mathrm{AC}^{2}+\mathrm{BD}^{2}$
$\mathrm{AB}^{2}+\mathrm{AB}^{2}+\mathrm{AB}^{2}+\mathrm{AB}^{2}=\mathrm{AC}^{2}+\mathrm{BD}^{2}$
$\mathrm{AB}^{2}+\mathrm{BC}^{2}+\mathrm{CD}^{2}+\mathrm{DA}^2=\mathrm{AC}^{2}+\mathrm{BD}^{2}$
Hence proved.
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