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python/数学课程/【辅助圆问题】定高定角.md

910 B

  • 定角:则做\triangle BAC的外接圆

  • 既然都有了外接圆,就需要有圆心有半径,设圆心O,连接OA=OB=OC=r

  • \because \angle BAC=60^{\circ} \therefore \angle BOC=120 ^ \circ

  • OBC垂线OEBCE点 则\angle EOC=60 ^{\circ},\angle ECO=30 ^{\circ}

    \therefore OE=\frac{1}{2}r,EC=\frac{\sqrt{3}}{2}r

    \therefore BC=\sqrt{3}r

    预求\triangle ABC的面积最小值,面积表示为 \frac{1}{2} BC \times AD 其中AD为定高,等于3,所以BC最小,则面积最小。 也就是\sqrt{3}r最小即可,也就是r最小。

    OA+OE>=AE>=AD \therefore r+\frac{1}{2}r>=3 \therefore r>=2 \therefore BC>=2\sqrt{3} S=\frac{1}{2}\times 2\sqrt{3}\times 3=3\sqrt{3}