8. 已知等差数列 $\left\{a_{n}\right\}$ 的公差为 2 , 若 $a_{1}, a_{3}, a_{4}$ 成等比数列, 则 $a_{2}=$
由题意知, $a_{3}=a_{1}+4, a_{4}=a_{1}+6$.

$\because a_{1}, a_{3}, a_{4}$ 成等比数列,

$\therefore a_{3}^{2}=a_{1} a_{4}, \quad \therefore\left(a_{1}+4\right)^{2}=\left(a_{1}+6\right) a_{1}$,

解得 $a_{1}=-8, \therefore a_{2}=-6$.