纯洁高雅
幼苗
共回答了16个问题采纳率：100% 举报
cos^2A+cos^2(A+60)-cosA*cos(A+60)
=1/2(1+c0s2A)+1/2[1+cos(2A+120)]-cosA*(cosAcos60-sinAsin60)
=1+1/2cos2A+1/2(cos2Acos120-sin2Asin120)-(cosA)^2cos60+1/2sin2Asin60
=1+1/2cos2A-1/4cos2A-1/2sin2Asin120-1/2(cosA)^2+1/2sin2Asin60
=1-1/4cos2A-1/4(1+cos2A)
=3/4-1/2cos2A
1年前
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