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利用公式cosα·cosβ=(1/2)[cos(α+β)+cos(α-β)] 得到COS(A+B)COS(A-B)=1/2[cos2A+cos2B]=1/3 因為cos2α=2cos^2α-1 COS(A+B)COS(A-B)=cos^2A-cos^2B-1=1/3 cos^2A-cos^2B=4/3
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