设S△ABC
a,b,c分别为∠A,B,C的对边
h_a为△ABC在边a上的高
R、r分别为△ABC外接圆，内切圆的半径
p为△ABC的半周长
r_a为△ABC与a边相切的旁切圆的半径
则有：
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\Large S_{△ABC}=\frac{1}{2}ah_a
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\Large S_{△ABC}=\frac{1}{2}bc \sin A
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\Large S_{△ABC}=\sqrt{p(p-a)(p-b)(p-c)}
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\Large S_{△ABC}=\frac{1}2r(a+b+c)=pr
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\Large S_{△ABC}=\frac{abc}{4R}
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\Large S_{△ABC}=2R^2 \sin A \sin B \sin C
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\Large S_{△ABC}=\frac{a^2 \sin B \sin C}{2 \sin (B+C)}
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\Large S_{△ABC}=\frac{1}2r_a(b+c-a)
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\Large S_{△ABC}=\frac12R^2(\sin 2A + \sin 2B + \sin 2C)