The distance between point $P$ and the origin, $O$, is $r$ units. The line $OP$ forms an angle of $\phi$ with the $x$-axis. Point $P'$ is the result of rotating point $P$ $\theta$ degrees counter-clockwise around the origin. The line $AP'$ forms a right angle with line $OP$.  **What is the length $OA$ in terms of $r$ and $\theta$?** [[☃ expression 1]]