cos36°-cos72°=?cos36-cos72=-2sin54sin(-18)这一步看不懂=2sin54sin18
3个回答

cos36-cos72

=cos(54-18)-cos(54+18)

=cos54cos18+sin54sin18-[cos54cos18-sin54sin18]

=2sin54sin18

=-2sin54sin(-18)

=2sin54sin18

=2cos36sin18

=2cos36sin18cos18/cos18

=cos36sin36/cos18

=sin72/(2cos18)

=sin72/(2sin72)=1/2