Use the fact that for all $\theta, \sin (-\theta)=-\sin \theta$ and $\cos (-\theta)=\cos \theta$ to prove that if a point $\mathbf{S}\left(x_1, y_1\right)$ has polar coordinates $(r, \theta)$ and a point $\mathbf{T}\left(x_2, y_2\right)$ has polar coordinates $(r,-\theta)$, then $x_1=x_2$, and $y_1=-y_2$.