△abc的形状为:直角三角形.cos^2B－sin^2A=cos^2C,cos^2B-cos^2C=sin^2A,(cosB+cosC)*(cosB-cosC)=sin^2A,利用和差化积,得2*cos[(B+C)/2]*cos[(B-C)/2]*(-2)*sin[(B+C)/2*sin[(B-C)/2]=4*sin^2(A/2)*cos^2(A/2),而,s...