∫xtan²xdx
=∫x(1/cos²x -1) dx
=∫x/cos²x dx -∫x dx
=∫ x d(tanx) - 0.5x²
= - 0.5x² + x *tanx - ∫tanx dx
= - 0.5x² + x *tanx + ln|cosx| +C