8. Given the function $f(x)=a x^{2}+b x(a b \neq 0)$, if $f\left(x_{1}\right)=f\left(x_{2}\right)\left(x_{1} \neq x_{2}\right)$, then the value of $f\left(x_{1}+x_{2}\right)$ is $\qquad$ .
8. 0 . Axis of symmetry $x=\frac{x_{1}+x_{2}}{2}=-\frac{b}{2 a}$, then $x_{1}+x_{2}=-\frac{b}{a}$, then $f\left(x_{1}+x_{2}\right)=f\left(-\frac{b}{a}\right)=0$.