I've been stuck trying to figure this one out for a while...
sin(2A) + sin(4A) = 0
Let $\displaystyle u = 2A$
$\displaystyle sin(u) + sin(2u) = 0$
$\displaystyle 2sin(u)cos(u) + sin(u) = 0$
$\displaystyle sin(u)(2cos(u)+1) = 0$
Either sin(u) = sin(2A) = 0 so A=0
or $\displaystyle 2cos(u) = -1 \rightarrow cos(u) = -\frac{1}{2}$
and so $\displaystyle u = \frac{2\pi}{3}$ thus $\displaystyle 2A = \frac{2\pi}{3}
\rightarrow A = \frac{\pi}{3}$
bear in mind this graph will repeat every $\displaystyle \pi$ radians instead of every $\displaystyle 2\pi$ radians