Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$(1,\pi)$
The point $(-1,0)$ lies on the $x$-axis a distance of $1$ unit from the pole (origin), so $r=1$. A ray with vertex at the pole through $(-1,0)$ forms an angle $\theta=\pi$ (counterclockwise) with the polar axis. Polar coordinates for this point can be given by $(1,\pi)$.