## Precalculus (6th Edition) Blitzer

The polar equation of $x=7$ that expresses $r$ in terms of $\theta$ is $r=\frac{7}{\cos \theta }$.
The relation between polar coordinates and rectangular coordinate $x$ can be expressed as below: $x=r\cos \theta$ (2) Substitute the value of $x\$ from equation (2) into equation (1) to get, \begin{align} & x=7 \\ & r\cos \theta =7 \\ & r=\frac{7}{\cos \theta } \end{align}