(15) Let $f(x)$ and $g(x)$ be odd and even functions defined on $\mathbf{R}$, respectively. When $x < 0$, $F(x) = f(x) g(x)$ is an increasing function on $(-\infty, 0)$, and $g(2) = 0$. Then the solution set of the inequality $f(x) g(x) < 0$ is $\qquad$
15 15) $\{x \mid x<-2$ or $0<x<2\}$