We have indeterminateness of type
oo/oo,
i.e. limit for the numerator is
n→∞lim(n(n+1)2)=∞and limit for the denominator is
n→∞lim(n+2)=∞Let's take derivatives of the numerator and denominator until we eliminate indeterninateness.
n→∞lim(n(n+2)(n+1)2)=
Let's transform the function under the limit a few
n→∞lim(n(n+2)(n+1)2)=
n→∞lim(dnd(n+2)dndn(n+1)2)=
n→∞lim(1−n21)=
n→∞lim(1−n21)=
1It can be seen that we have applied Lopital's rule (we have taken derivatives with respect to the numerator and denominator) 1 time(s)