How to verify this trigonometric identity? Please help,
$9\cos^2 B − 9\sin^2 B = 18\cos^2 B − 9$
Thanks

\begin{align*}
\text{LHS} &= 9\cos^2 B − 9\sin^2 B \\ 
&= 9(\cos^2 B-\sin^2 B) \\ 
&= 9(\cos^2 B - (1-\cos^2 B)) \\
&= 9(2\cos^2 B- 1) \\
&= 18\cos B - 9 \\
&= \text{RHS}.
\end{align*}
