2022年高考数学新高考Ⅱ-16 |
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2022-12-16 17:32:22 |
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(5分)已知直线$l$与椭圆$\dfrac{x^2}{6}+\dfrac{y^2}{3}=1$在第一象限交于$A$,$B$两点,$l$与$x$轴、$y$轴分别相交于$M$,$N$两点,且$\vert MA\vert =\vert NB\vert$,$\vert MN\vert =2\sqrt{3}$,则$l$的方程为 $x+\sqrt{2}y-2\sqrt{2}=0$ . 分析:设$A(x_{1}$,$y_{1})$,$B(x_{2}$,$y_{2})$,线段$AB$的中点为$E$,可得$k_{OE}\cdot k_{AB}=\dfrac{{y}_{1}+{y}_{2}}{{x}_{1}+{x}_{2}}\cdot \dfrac{{y}_{2}-{y}_{1}}{{x}_{2}-{x}_{1}}=-\dfrac{1}{2}$,设直线$l$的方程为:$y=kx+m$,$k < 0$,$m > 0$,$M(-\dfrac{m}{k}$,$0)$,$N(0,m)$,可得$E(-\dfrac{m}{2k}$,$\dfrac{m}{2})$,$k_{OE}=-k$,进而得出$k$,再利用$\vert MN\vert =2\sqrt{3}$,解得$m$,即可得出$l$的方程. 解:设$A(x_{1}$,$y_{1})$,$B(x_{2}$,$y_{2})$,线段$AB$的中点为$E$, 由$\dfrac{{x}_{1}^{2}}{6}+\dfrac{{y}_{1}^{2}}{3}=1$,$\dfrac{{x}_{2}^{2}}{6}+\dfrac{{y}_{2}^{2}}{3}=1$, 相减可得:$\dfrac{{y}_{2}^{2}-{y}_{1}^{2}}{{x}_{2}^{2}-{x}_{1}^{2}}=-\dfrac{1}{2}$, 则$k_{OE}\cdot k_{AB}=\dfrac{{y}_{1}+{y}_{2}}{{x}_{1}+{x}_{2}}\cdot \dfrac{{y}_{2}-{y}_{1}}{{x}_{2}-{x}_{1}}=\dfrac{{y}_{2}^{2}-{y}_{1}^{2}}{{x}_{2}^{2}-{x}_{1}^{2}}=-\dfrac{1}{2}$, 设直线$l$的方程为:$y=kx+m$,$k < 0$,$m > 0$,$M(-\dfrac{m}{k}$,$0)$,$N(0,m)$, $\therefore E(-\dfrac{m}{2k}$,$\dfrac{m}{2})$,$\therefore k_{OE}=-k$, $\therefore -k\cdot k=-\dfrac{1}{2}$,解得$k=-\dfrac{\sqrt{2}}{2}$, $\because \vert MN\vert =2\sqrt{3}$,$\therefore$$\sqrt{\dfrac{{m}^{2}}{{k}^{2}}+{m}^{2}}=2\sqrt{3}$,化为:$\dfrac{{m}^{2}}{{k}^{2}}+m^{2}=12$. $\therefore 3m^{2}=12$,$m > 0$,解得$m=2$. $\therefore l$的方程为$y=-\dfrac{\sqrt{2}}{2}x+2$,即$x+\sqrt{2}y-2\sqrt{2}=0$, 故答案为:$x+\sqrt{2}y-2\sqrt{2}=0$. 点评:本题考查了椭圆的标准方程及其性质、两点之间的距离公式,考查了推理能力与计算能力,属于中档题.
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