# How do you use the double angle or half angle formulas to express sin x/2 in terms of cos x?

cos 2x = 1 - 2 ${\sin}^{2}$x $\implies$ 2${\sin}^{2}$ x = 1 - cos2x $\implies$${\sin}^{2}$x/2 = (1- cosx ) /2$\implies$ sin x/2 = sqrt 1/2(1 - cos x).