cos20度-cos40度-cos80度等于?

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cos20度-cos40度-cos80度等于?cos20度-cos40度-cos80度等于?cos20度-cos40度-cos80度等于?cos20-cos(60-20)-cos(60+20)cos2

cos20度-cos40度-cos80度等于?
cos20度-cos40度-cos80度等于?

cos20度-cos40度-cos80度等于?
cos20-cos(60-20)-cos(60+20)
cos20-cos20cso60-sin20sin60-cos20cos60+sin20sin60
cos20-2*cos20cos60=0

0

cos20-cos(60-20)-cos(60+20)
cos20-cos20cso60-sin20sin60-cos20cos60+sin20sin60
cos20-2*cos20cos60=0

cos20-cos40-cos80
=-2sin30sin(-10)-cos80
=sin10-sin10
=0