## Precalculus (6th Edition) Blitzer

The solutions of the equation is $\theta =\pi +2n\pi$, where n is any integer.
\begin{align} & 7\cos \theta +9=-2\cos \theta \\ & 7\cos \theta +2\cos \theta =-9 \\ & 9\cos \theta =-9 \\ & \cos \theta =-1 \end{align} $\cos \pi =-1$, and the solution of $\cos \theta =-1$ in [0, $2\pi$ ) is: $\theta =\pi$ The period of the tangent function is $2\pi$, and the solutions are given by: $\theta =\pi +2n\pi$, where n is any integer.