2022 SSMO Team Round Problems/Problem 7

Problem

Let $\cos(2A)$, $\cos(2B)$, and $\cos(2C)$ be the not necessarily distinct roots of a monic cubic $f$. Given that $f(1)= \frac{17}{30}$, the value of $\sin(A)\sin(B)\sin(C)$ can be expressed as $\frac{\sqrt{m}}{n},$ with $m$ squarefree. Find $m+n$.

Solution