设圆的半径为R,角AOB为a,角AOC为bAB^2=AO^2+BO^2-2AO BOcosa=2R^2-2R^2cosaAC^2=AO^2+CO^2-2AO COcosb=2R^2-2R^2cosbAO*BC=AO*(BO+OC)=AO*BO-AO*CO=R^2cosa-R^2cosb=(AC^2-AB^2)/2=8