By definition - $q(n)$ is a negligible function if for every positive integer $c>0$ there exist an integer $N_c$ such that for all $x>N_c$ :
- $q(n)<1/x^c$
So for the function $1/x$, if we take the positive integer $c=1$ then we need to find some $N_c$ such that for every $x>N_c$:
- $1/x<1/x$
$=>$ Definitely such $N_c$ does not exist then $1/x$ is not a negligible function.
Is that sufficient?