△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,sin[(B+C)]=cos(A/2),sin[(B+C)/2]=cos(A/2),则有, -cos[(B-C)/2]*sin[(B-C)/2]=sin(A/2)*cos(A/2), -sin(B-C)=sinA, sin(C-B)=sinA, C...
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