The question is : Find the general solution of this equation $ \sin (5\phi)-\sin \phi=\sin (2\phi) $ I tried to expand $\sin (5\phi)$ and $\sin (2\phi)$, so the equation only contains $\cos\phi$ and $\sin \phi$.
But I can't make it into a form like $\sin(\phi+a)=n$ to get the general solution.
There's another question like this :
Find the general solution of this equation $ \sin 2x+\sin 4x=\cos 2x+\cos 4x $