当arctanx>0,
[x^(-2) ln|arctanx|]'
=[x^(-2) lnarctanx]'
=-2x^(-3)×(1/arctanx)×(arctanx）'
=[-2x^(-3)/arctanx] ×1/(1+x²)
= -2x^(-3)/[(1+x²)arctanx]
当arctanx