8. Given that the opposite number of a root of $x^{2}-4 x+b=0$ is a root of $x^{2}+4 x-b=0$, the positive root of $x^{2}+$ $b x-4=0$ is $\qquad$ .
8. 2 .

Let $x_{0}$ be a root of $x^{2}-4 x+b=0$, then $x_{0}^{2}-4 x_{0}+b=0$. Also, $-x_{0}$ is a root of $x^{2}+4 x-b=0$, then $x_{0}^{2}-4 x_{0}-b=0$. Therefore, $b=0$, the equation $x^{2}+b x-4=0$ becomes $x^{2}-4=0$, the positive root is 2.