y=ln(x^2+3x+2)
y'=(2x+3)/(x^2+3x+2)=1/(x+1)+1/(x+2)
因此y的的n阶导数为（-1）^（n-1）*(n-1)!/(x+1)^n+(-1)^(n-1)*(n-1)!/(x+2)^n