2. Given the equation in terms of $x$
$$
a x^{2}=k x+b(a b k \neq 0)
$$

with roots $x_{1} 、 x_{2}$, and
$$
k x+b=0(k b \neq 0)
$$

with root $x_{3}$. The quantitative relationship satisfied by $x_{1} 、 x_{2} 、 x_{3}$ is $\qquad$
2. $\frac{1}{x_{1}}+\frac{1}{x_{2}}=\frac{1}{x_{3}}$.

From $a x^{2}-k x-b=0$, we get
$x_{1}+x_{2}=\frac{k}{a}, x_{1} x_{2}=-\frac{b}{a}$.
Also, since $x_{3}=-\frac{b}{k}$, we have
$x_{3}\left(x_{1}+x_{2}\right)=-\frac{b}{a}$.
Thus, $x_{3}\left(x_{1}+x_{2}\right)=x_{1} x_{2}$, which means
$$
\frac{1}{x_{1}}+\frac{1}{x_{2}}=\frac{1}{x_{3}} \text {. }
$$