## Precalculus (6th Edition) Blitzer

$-\frac{1}{4}$.
The sine function is odd. So, $f\left( -a \right)=-f\left( a \right)$ Substitute into the main expression: \begin{align} & f\left( a \right)+2f\left( -a \right)=f\left( a \right)-2f\left( a \right) \\ & =-f\left( a \right) \\ & =-\frac{1}{4} \end{align} Thus, the value of the given expression is, $-\frac{1}{4}$.