若sin(π3−α)=13,则cos(5π6−α)的值为(  ) A.13 B.−13 C.223 D.−223

问题描述:

sin(

π
3
−α)=
1
3
,则cos(
6
−α)
的值为(  )
A.
1
3

B.
1
3

C.
2
2
3

D.
2
2
3

(

π
3
−α)+(α+
π
6
)=
π
2

sin(
π
3
−α)=sin[
π
2
−(α+
π
6
)]=
1
3
,即cos(α+
π
6
)=
1
3

又∵(α+
π
6
)+(
6
−α
)=π,
cos(
6
−α)
=cos[π−(α+
π
6
)]
=-cos(α+
π
6
)=-
1
3

故选:B