等比数列 $\left\{a_{n}\right\}$ 中, $a_{2}, a_{6}$ 是方程 $x^{2}-34 x+64=0$ 的两根, 则 $a_{4}$ 等于 $(\quad)$
解析 $\because a_{2}+a_{6}=34, a_{2} \cdot a_{6}=64, \therefore a_{4}^{2}=64$,

$\because a_{2}>0, \quad a_{6}>0, \quad \therefore a_{4}=a_{2} q^{2}>0, \quad \therefore a_{4}=8$.