1. If the product of the maximum and minimum elements of the set of real numbers $\{3,6,9, x\}$ equals the sum of all elements in the set, then the value of $x$ is $\qquad$ .
If $x \geqslant 9 \Rightarrow 3 x=3+6+9+x=18+x \Rightarrow x=9$, contradiction; If $x \leqslant 3 \Rightarrow 9 x=18+x$ $\Rightarrow x=\frac{9}{4}$; If $3<x<9 \Rightarrow 27=18+x \Rightarrow x=9$, contradiction. Therefore, $x=\frac{9}{4}$.