左边=（ln(x^2+1)+x*(1/x^2+1)*2x）dx=(x*ln(x^2+1)+2x^3/(x^2+1)
)*(1/x)dx,
右边d(lnx)=(1/x)dx,
所以应该填x*ln(x^2+1)+2x^3/(x^2+1)