Let $P(x,y,z)$ be a point in the first octant whose projection on the $xy$–plane is $Q$. Let $OP=\gamma$; the angle between $OQ$ and the positive $x$–axis be $\theta$; and the angle between $OP$ and the positive $z$–axis be $\phi$ (with $O$ the origin). The distance of $P$ from the $x$–axis is