(5) If the set $A=\left\{x \mid a x^{2}+3 x+1=0, x \in \mathbf{R}\right\}$ contains at most one element, then the range of real number $a$ is $\qquad$.
(5. $\left\{a \mid a=0\right.$, or $\left.a \geqslant \frac{9}{4}\right\}$