We have indeterminateness of type
oo/oo,
i.e. limit for the numerator is
x→∞lim(x−atan(x))=∞and limit for the denominator is
x→∞limx=∞Let's take derivatives of the numerator and denominator until we eliminate indeterninateness.
x→∞lim(xx−atan(x))=
x→∞lim(dxdxdxd(x−atan(x)))=
x→∞lim(1−x2+11)=
x→∞lim(1−x2+11)=
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)