已知双曲线 $M:\dfrac{x^{2}}{a^{2}}-\dfrac{y^{2}}{b^{2}}=1$($a>0,b>0$)的左右焦点为 $F_1,F_2$,$P$ 为双曲线右支上一点,$Q$ 为 $\triangle PF_1F_2$ 的内心,其满足 $2\overrightarrow{QF_1}+3\overrightarrow{QF_2}=4\overrightarrow{PQ}$,则 $M$ 的离心率为
解:延长 $QF_1$ 至 $A$ 使得 $QA=2QF_1$,
延长 $QF_2$ 至 $B$ 使得 $QB=3QF_2$,
延长 $QP$ 至 $C$ 使得 $QC=4QP$。
$\because 2\overrightarrow{QF_1}+3\overrightarrow{QF_2}=4\overrightarrow{PQ}$
$\therefore \overrightarrow{QA}+\overrightarrow{QB}+\overrightarrow{QC}=\vec{0}$
$\therefore Q$ 为 $\triangle ABC$ 重心
$\therefore S_{\triangle AOC}=S_{\triangle AOB}=S_{\triangle BOC}$
$\therefore 8S_{\triangle QPF_1}=6S_{\triangle QF_1F_2}=12S_{\triangle QPF_2}$
$\because Q$ 为 $\triangle PF_1F_2$ 内心
$\therefore 4PF_1=3F_1F_2=6PF_2$
$\therefore PF_1=\dfrac{3}{2}c$,$PF_2=c$
$\therefore PF_1-PF_2=\dfrac{1}{2}c=2a$
$\therefore e=\dfrac{c}{a}=4$
另解:设 $|PF_2|=d$,则 $|PF_1|=2a+d$。
$\because Q$ 为 $\triangle PF_1F_2$ 内心
$\therefore 2c\cdot\overrightarrow{QP}+d\cdot\overrightarrow{QF_1}+(2a+d)\cdot\overrightarrow{QF_2}=\vec{0}$
$\because 2\overrightarrow{QF_1}+3\overrightarrow{QF_2}=4\overrightarrow{PQ}$,即 $\overrightarrow{QP}=-\dfrac{2\overrightarrow{QF_1}+3\overrightarrow{QF_2}}{4}$
$\therefore 2c\cdot\left( -\dfrac{2\overrightarrow{QF_1}+3\overrightarrow{QF_2}}{4} \right)+d\cdot\overrightarrow{QF_1}+(2a+d)\cdot\overrightarrow{QF_2}=\vec{0}$
$\therefore (d-c)\overrightarrow{QF_1}+\left( 2a+d-\dfrac{3}{2}c \right)\overrightarrow{QF_2}=\vec{0}$
$\therefore d-c=0$,$2a+d-\dfrac{3}{2}c=0$
$\therefore 4a=c$
$\therefore e=\dfrac{c}{a}=4$
解:如图所示,在焦点三角形中,延长 $PQ$ 交 $F_1F_2$ 于点 $A$。
因为 $Q$ 为 $\triangle F_1F_2P$ 的内心,所以有 $\dfrac{|PF_1|}{|AF_1|}=\dfrac{|PQ|}{|QA|}$,$\dfrac{|PF_1|}{|AF_1|}=\dfrac{|PF_2|}{|AF_2|}$。
$\overrightarrow{PQ}=\dfrac{|PF_1|}{|AF_1|}\overrightarrow{QA} \Rightarrow \overrightarrow{PQ}=\dfrac{|PF_1|}{|AF_1|}(\overrightarrow{QF_1}+\overrightarrow{F_1A})$
$\Rightarrow |AF_1|\cdot\overrightarrow{PQ}=|PF_1|\cdot\overrightarrow{QF_1}+|PF_1|\cdot\overrightarrow{F_1A}$
$\Rightarrow |AF_1|\cdot\overrightarrow{PQ}=|PF_1|\cdot\overrightarrow{QF_1}+|PF_1|\cdot\dfrac{|AF_1|}{|PF_1|}\left( \dfrac{|AF_1|}{|AF_1|}\overrightarrow{F_1A}+\overrightarrow{F_1A} \right)$
$\Rightarrow |AF_1|\cdot|F_2F_1|\cdot\overrightarrow{PQ}=|PF_1|\cdot|F_2F_1|\cdot\overrightarrow{QF_1}+|PF_1|\cdot|AF_1|\cdot\overrightarrow{F_1Q}+|PF_1|\cdot|AF_1|\cdot\overrightarrow{QF_2}$
$\Rightarrow |AF_1|\cdot|F_2F_1|\cdot\overrightarrow{PQ}=|PF_1|\cdot|F_2F_1|\cdot\overrightarrow{QF_1}+|PF_1|\cdot|AF_1|\cdot\overrightarrow{F_1Q}+|PF_1|\cdot|AF_1|\cdot\overrightarrow{QF_2}$
$\Rightarrow |AF_1|\cdot|F_2F_1|\cdot\overrightarrow{PQ}=|PF_2|\cdot|AF_2|\cdot\overrightarrow{QF_1}+|PF_1|\cdot|AF_1|\cdot\overrightarrow{QF_2}$
$\Rightarrow |F_2F_1|\cdot\overrightarrow{PQ}=\dfrac{|PF_2|\cdot|AF_2|}{|AF_1|}\cdot\overrightarrow{QF_1}+|PF_1|\cdot\overrightarrow{QF_2} \Rightarrow |F_2F_1|\cdot\overrightarrow{PQ}=\dfrac{|PF_2|\cdot|PF_1|}{|AF_1|}\cdot\overrightarrow{QF_1}+|PF_1|\cdot\overrightarrow{QF_2}$
$\Rightarrow |F_2F_1|\cdot\overrightarrow{PQ}=|PF_2|\cdot\overrightarrow{QF_1}+|PF_1|\cdot\overrightarrow{QF_2}$,因为 $2\overrightarrow{QF_1}+3\overrightarrow{QF_2}=4\overrightarrow{PQ}$,
所以有 $|F_2F_1|=4k$,$|PF_1|=3k$,$|PF_2|=2k$,
因此 $M$ 的离心率为 $e=\dfrac{c}{a}=\dfrac{2c}{2a}=\dfrac{|F_1F_2|}{|PF_1|-|PF_2|}=4$。