lim(x->+∞) y(x)=lim(x->+∞) e^(-x)*∫(0,x) e^t*f(t)dt=lim(x->+∞) [∫(0,x) e^t*f(t)dt]/e^x=lim(x->+∞) [e^x*f(x)]/e^x=lim(x->+∞) f(x)=1
