makes sense to me, but i don't understand what it has to do with triangle inequality, it is obvious that $\left|x^2+1\right| \ge \left| x^2 \right|$
– gt6989bMay 27 '22 at 03:56
Writing $\left|\frac x{x^2+1}\right|\leq\frac 1{|x|}$ does not help. You need to write it as $\left|\frac x{x^2+1}\right|\leq |x|$, using $x^2+1\geq 1$.
– PythagorasMay 27 '22 at 04:05