2022年高考数学浙江17 |
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2022-12-16 20:48:01 |
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(4分)设点$P$在单位圆的内接正八边形$A_{1}A_{2}\ldots A_{8}$的边$A_{1}A_{2}$上,则$\overrightarrow{P{A}_{1}}^{2}+\overrightarrow{P{A}_{2}}^{2}+\ldots +\overrightarrow{P{A}_{8}}^{2}$的取值范围是 $[12+2\sqrt{2}$,$16]$ . 分析:以圆心为原点,$A_{7}A_{3}$所在直线为$x$轴,$A_{5}A_{1}$所在直线为$y$轴,建立平面直角坐标系,求出正八边形各个顶点坐标,设$P(x,y)$,进而得到$\overrightarrow{P{A}_{1}}^{2}+\overrightarrow{P{A}_{2}}^{2}+\ldots +\overrightarrow{P{A}_{8}}^{2}=8(x^{2}+y^{2})+8$,根据点$P$的位置可求出$x^{2}+y^{2}$的范围,从而得到$\overrightarrow{P{A}_{1}}^{2}+\overrightarrow{P{A}_{2}}^{2}+\ldots +\overrightarrow{P{A}_{8}}^{2}$的取值范围. 解:以圆心为原点,$A_{7}A_{3}$所在直线为$x$轴,$A_{5}A_{1}$所在直线为$y$轴,建立平面直角坐标系,如图所示,
则$A_{1}(0,1)$,${A}_{2}(\dfrac{\sqrt{2}}{2},\dfrac{\sqrt{2}}{2})$,$A_{3}(1,0)$,${A}_{4}(\dfrac{\sqrt{2}}{2},-\dfrac{\sqrt{2}}{2})$,$A_{5}(0,-1)$,${A}_{6}(-\dfrac{\sqrt{2}}{2},-\dfrac{\sqrt{2}}{2})$,$A_{7}(-1,0)$,${A}_{8}(-\dfrac{\sqrt{2}}{2},\dfrac{\sqrt{2}}{2})$, 设$P(x,y)$, 则$\overrightarrow{P{A}_{1}}^{2}+\overrightarrow{P{A}_{2}}^{2}+\ldots +\overrightarrow{P{A}_{8}}^{2}=\vert PA_{1}\vert ^{2}+\vert PA_{2}\vert ^{2}+\vert PA_{3}\vert ^{2}+\vert PA_{4}\vert ^{2}+\vert PA_{5}\vert ^{2}+\vert PA_{6}\vert ^{2}+\vert PA_{7}\vert ^{2}+\vert PA_{8}\vert ^{2}=8(x^{2}+y^{2})+8$, $\because \cos 22.5^\circ \leqslant \vert OP\vert \leqslant 1$,$\therefore$$\dfrac{1+\cos 45^\circ }{2}\leqslant {x}^{2}+{y}^{2}\leqslant 1$, $\therefore$$\dfrac{2+\sqrt{2}}{4}\leqslant {x}^{2}+{y}^{2}\leqslant 1$, $\therefore 12+2\sqrt{2}\leqslant 8(x^{2}+y^{2})+8\leqslant 16$, 即$\overrightarrow{P{A}_{1}}^{2}+\overrightarrow{P{A}_{2}}^{2}+\ldots +\overrightarrow{P{A}_{8}}^{2}$的取值范围是$[12+2\sqrt{2}$,$16]$, 故答案为:$[12+2\sqrt{2}$,$16]$.
点评:本题主要考查了平面向量数量积的运算和性质,考查了学生分析问题和转化问题的能力,属于中档题.
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