$\displaystyle
sin(x+50)=2cosx
$
It's weird that this question doesn't have any special angle or any sort, I don't know how to solve this question with $\displaystyle 50^\circ$ as the angle!!
It is the inverse function to tangent. tan(arctan(x))= x and arctan(tan(x))= x as long as x is between $\displaystyle -pi/2$ and $\displaystyle \pi/2$. Also called $\displaystyle tan^{-1}(x)$ which must not be confused with $\displaystyle \frac{1}{tan(x)}= cot(x)$.