y=e^x-(1+x),y(0)=0,y'=e^x-1>=0 (x>0)
e^x>肺^k/k! (n)->d^ne^x=e^x>1+x->e^x>肺^k/k! (n+1) (x>0)
x^n/肺^k/k! (n+2)<x^n/e^x<x^n/肺^k/k! (n+1)->1/x^2<f<1/x->0