f=ln(x+(1+x^2)^(1/2)),求导：
f'=1/(1+x^2)^(1/2)=(1+x^2)^(-1/2)
=1+(-1/2)x^2+(-1/2)(-1/2-1)x^4+(-1/2)(-1/2-1)(-1/2-2)x^6+...|x|