Question

Medals for anybody who can help me with all my precal quiz questions

Answer 1

Use the formula:
\[\sin(ax)\sin(bx)=\frac{1}{2}(\cos((a-b)x)-\cos((a+b)x)))\]
So:
\[\sin(9x)\sin(4x)=\frac{1}{2}(\cos((9-4)x)-\cos((9+4)x))=\frac{1}{2}(\cos(5x)-\cos(13x))\]