Arctan is just the inverse of tan. All functions of the form

trig(arctrig(x))

Can be simplified into an algebraic equation that involves division and square roots.To do that, draw a right triangle, and use the triangle SOH-CAO-TOA definitions. In this case, define an angle θ, and define the opposite side to be x, and the adjacent side to be sqrt(3), so that tan(θ) = x/sqrt(3), and arctan(x/sqrt(3)) = θ. Then find the hypotenuse using the Pythagorean theorem, and find sin(θ). This will be the expression you want.

My explanation might have sucked so tell me if this made sense xD