求导就用链式法则一步步来,
[ln(1+x^2)]^2 '
= 2ln(1+x^2) * [ln(1+x^2)] '
= 2ln(1+x^2) * 1/(1+x^2) * (1+x^2)'
= 2ln(1+x^2) * 1/(1+x^2) * 2x
=4x/(1+x^2) *ln(1+x^2)