由余弦公式,a^2+b^2+c^2-2ab*cosC = c^2,由正弦公式.a/sinA=b/sinB=c/sinC=2R,其中R为外接圆半径.所以 sinA=a/2R,sinB=b/2R,sinC=c/2R,代入原式,得到sin^2A+sin^B-2sinAsinBcosC=(a/2R)^2+(b/2R)^2-2(a/2R)(b/2R)cos...