y=ln(1+x^2)的二阶导数,
问题描述：
y=ln(1+x^2)的二阶导数,
答
y' = (1+x²)'/(1+x²) = 2x/(1+x²)
y" = [(1+x²)(2x)' - (1+x²)'(2x)/(1+x²)²
= 2(1-x²)/(1+x²)²