## Precalculus (6th Edition) Blitzer

To test whether the graph of a polar equation may be symmetric with respect to the polar axis $\left( x\text{-axis} \right)$, replace $\theta$ with $-\theta$.
When $\theta$ is replaced with $-\theta$, if the resultant polar equation is equivalent to the given polar equation then a graph of a polar equation exhibits symmetry with respect to the polar axis $\left( x\text{-axis} \right)$.