cos75°cos15°-sin255°sin165°的值是( ) A.12 B.−12 C.32 D.0
问题描述:
cos75°cos15°-sin255°sin165°的值是( )
A.
1 2
B. −
1 2
C.
3
2
D. 0
答
cos75°cos15°-sin255°sin165°
=cos75°cos15°-sin(180°+75°)sin(180°-15°)
=cos75°cos15°+sin75°sin15°
=cos(75°-15°)
=cos60°
=
.1 2
故选A