e^x
=1 + x + x^2/2!+ x^3/3!+ x^4/4!+ x^5/5!+...+x^n/n!+...
f(x)=xe^x
=x + x^2 + x^3/2!+ x^4/3!+ x^5/4!+ x^6/5!+...+x^(n+1)/n!+...