cos75度*cos15度-sin255度*sin15度=？

Question

步骤具体

Answer 1

cos75=sin15 ,sin255=sin(360-105),
由正弦函数周期性的sin（360-105）=-sin105=-sin（90+15）=-cos15
原式为sin15cos15+cos15sin15=2sin15cos15=sin30=1 /2

Answer 2

cos75°*cos15°-sin255°*sin15°
=sin15°cos15°-sin(180°+75°)sin15°
=sin15°cos15°+sin75°sin15°
=sin15°cos15°+cos15°sin15°
=2sin15°cos15°
=sin30°=1/2

Answer 3

cos75度*cos15度-sin255度*sin15度=cos75度*cos15度-sin75度*sin15度=cos（75+15）度=0