cos²79°+cos²41°-2cos79°*c - 知识问答

cos²79°+cos²41°-2cos79°*cos41°*cos120°的值为

2个回答

(1+cos158)/2 +(1+cos82)/2 +cos79cos41
=1-cos22 /2 +cos82/2 +cos79cos41
=1+1/2(cos82-cos22)+cos79cos41
=-sin52sin30+cos79cos41+1
=-sin52 /2 +1/2(cos120+cos38) +1
=-sin52 /2+cos38/2 +3/4
=3/4

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