[tex] \Big(\sin^2(ax)\Big)' = \\ \\ =\left[\Big(\sin(ax)\Big)^2\right]' = \\ \\ = 2 \cdot\Big(\sin(ax)\Big)^1 \cdot \Big(\sin(ax)\Big)' = \\ \\ = 2\cdot \sin(ax) \cdot \cos(ax) \cdot (ax)' = \\ \\ = 2\cdot \sin(ax)\cdot\cos(ax) \cdot a = \\ \\ =2a\cdot \sin(ax)\cdot \cos(ax) [/tex]