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正弦定理よりb/sin(Aπ/180)=AB/sin(Cπ/180)
AB=bsin(Cπ/180)/sin(Aπ/180)
∴△ABC=(1/2)BC・ABsin｛(180-A-C)π/180｝
=b^2sin(Cπ/180)sin｛(180-A-C)π/180｝/｛2sin(Aπ/180)｝