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4sin^2((B+C)/2)-cos2A=7/2, A+B+C=180度, B+C=180-A, (B+C)/2=90-A/2, sin[(B+C)/2]=sin(90-A/2)=cos(A/2), sin^2[(B+C)/2]=cos^2(A/2), 即有, 4*cos^2(A/2)-cos2A=7/2, 而,2cos^2(A/2)-1=cosA, ∴4cos^2(A/2)-2=2cosA, 4cos^2(A/2)-2-cos2A=7/2-2, 2cosA-(2cos^2(A)-1=7/2-2, 4cos^2(A)-4cosA+1=0, (2cosA-1)^2=0, 2cosA=1, co... |
