## Calculus (3rd Edition)

$$r=\frac{z}{\cos\theta +\sin\theta} .$$
Since $$x=r\cos \theta,\quad y=r\cos \theta, \quad z=z,$$ then $z=x+y$ takes the form $$z=r\cos\theta +r\sin\theta\Longrightarrow r=\frac{z}{\cos\theta +\sin\theta} .$$