[m]sin^4x=(\frac{1-cos4x}{2})^2=\frac{1-2cos4x+cos^24x}{4}=\frac{1-2cos4x+\frac{1+cos8x}{2}}{4}=\frac{3}{8}-\frac{1}{2}cos4x+\frac{1}{8}cos8x[/m]
[m] ∫ sin^22xdx= ∫ (\frac{3}{8}-\frac{1}{2}cos4x+\frac{1}{8}cos8x)dx=\frac{3}{8}x-\frac{1}{8}sin4x+\frac{1}{64}(-sin8x)+C[/m]