化简:sin21°cos81°-cos21°sin81°=( ) A.32 B.−32 C.12 D.−12
问题描述:
化简:sin21°cos81°-cos21°sin81°=( )
A.
3
2
B. −
3
2
C.
1 2
D. −
1 2
答
sin21°cos81°-cos21°sin81°
=sin21°cos(90°-9°)-cos21°sin(90°-9°)
=sin21°sin9°-cos21°cos9°
=-(cos21°cos9°-sin21°sin9°)
=-cos(21°+9°)
=-cos30°
=-
.
3
2
故选B