可以拆开来算，答案如图所示
∫x(tanx)^2 dx=∫x[(secx)^2 -1] dx=-(1/2)x^2 +∫x dtanx=-(1/2)x^2 +xtanx -∫tanx dx=-(1/2)x^2 +xtanx +ln|cosx| +C
