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

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