sin75°cos105°-sin105°sin15°的值等于

问题描述:

sin75°cos105°-sin105°sin15°的值等于

原式=sin75cos(180-75)-sin(180-75)sin(90-75)
=sin75(-cos75)-sin75cos75
=-sin75cos75-sin75cos75
=-2sin75cos75
=-sin150
=-1/2