Answer
(a) $F(x)=\frac{1}{2}cos(x)-\frac{1}{2}cos(2x)$
(b) See graph (red).
Work Step by Step
(a) $F(x)=sin(3x)sin(x)=\frac{1}{2}cos(\frac{3x-x}{2})-\frac{1}{2}cos(\frac{3x+x}{2})=\frac{1}{2}cos(x)-\frac{1}{2}cos(2x)$
(b) See graph (red).
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