Answer
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 $.
Work Step by Step
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 $\left( x\text{-axis} \right)$.
