# Chapter 4 - Section 4.2 - Trigonometric Functions: The Unit Circle - Exercise Set - Page 550: 103

$-\frac{1}{4}$.

#### Work Step by Step

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, $-\frac{1}{4}$.

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