$$\int (senx.cosx)².dx$$ $$\frac{1}{4}\int sen²2x$$ $$\frac{1}{8}\int (1-cos4x)dx$$ $$\frac{1}{8}[x-\int cos4x.dx]$$ $$\frac{1}{8}[x-\frac{sen4x}{4}]+c$$ $$\frac{1}{32}[4x-sen4x]+c$$