1-cos^2A-cos^2B-cos^2C=1-cos^2A-（cos^2B+cos^2C）
=1-cos^2A-[(1+cos2B)/2+(1+cos2C)/2]=1-cos^2A-(cos2B+cos2C)/2-1
=-cos^2A-cos(B+C)cos(B-C)=-cos^2A+cosAcos(B-C)
=cosA[-cosA+cos(B-C)]=cosA[cos(B+C)+cos(B-C)]=2cosAcosBcosC
∴1-cos^2A-cos^2B-cos^2C=2cosAcosBcosC
即1-cos^2A-cos^2B-cos^2C-2cosAcosBcosC=0