根号2/2(cos75°-sin75°)
=根号2/2(cos(30°+45°)-cos15°)
=根号2/2(cos(30°+45°)-cos(45°-30°))
=根号2/2(cos30°cos45°-sin30°sin45°-(cos30°cos45°+sin30°sin45°))
=根号2/2(cos30°cos45°-sin30°sin45°-cos30°cos45°-sin30°sin45°)
=根号2/2(-2sin30°sin45°)
=根号2/2(-2*1/2 *根号2/2)
=-1/2