Question

cos72度—cos36度=?,要过程.

Answer 1

cos72—cos

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

=[cos54cos18-sin54sin18]-cos54cos18+sin54sin18

=-2sin54sin18

=-2cos36sin18

=-2cos36sin18cos18/cos18

=-cos36sin36/cos18

=-sin72/(2cos18)

=-sin72/(2sin72)

=-1/2