dy/dx=(xexp(-x))'f'(xexp(-x))=(1-x)exp(-x)f'(xexp(-x)),d^2y/dx^2=((1-x)exp(-x))'f'(xexp(-x))+(1-x)exp(-x)(xexp(-x))'f''(xexp(-x))=(x-2)exp(-x)f'(xexp(-x))+(1-x)^2*exp(-2x)f''(xexp(-x)).
