# If the projection of $\overrightarrow{a}$ on $\overrightarrow{b}$ and the projection of $\overrightarrow{b}$ on $\overrightarrow{a}$ are equal then the angle between $\overrightarrow{a}+\overrightarrow{b}$ and $\overrightarrow{a}-\overrightarrow{b}$ is
$\begin{array}{1 1}(1)\frac{\pi}{2} &(2)\frac{\pi}{3}\\(3)\frac{\pi}{4}&(4)\frac{2\pi}{3}\end{array}$