证明：利用正弦定理a/(sina)=b/(sinb)=c/(sinc)=2R,就有：a^2=4R^2sin^2Ab^2=4R^2sin^2Bc^2=4r^2sin^2C(a^2-b^2)=4R^2(sin^2A-sin^2B)=4R^2(1-cos^2A-1+cos^2B)=4R^2(cos^2B-cos^2A)=4R^2(cosA+cosB)(cosB-cosA)……...