a/sinA=b/sinB=c/sinC=1/t则:sinA=at sinB=bt sinC=ct 代入sin^2A=sin^2B+sinBsinC+sin^2C(a^2t^2)=(b^2+bc+c^2)t^2a^2=b^2+bc+c^2a^2-(b^2+c^2)=bc余弦定理:a^2=b^2+c^2-2bccosA所以:b^2+c^2-2bccosA-(b^2+c^2)=bc-2cosA=1cosA=-1/2A=120
