已知sin(a+π6)=13,则cos(2a-2π3)的值是(  ) A.79 B.13 C.-13 D.-79

问题描述:

已知sin(a+

π
6
)=
1
3
,则cos(2a-
3
)的值是(  )
A.
7
9

B.
1
3

C. -
1
3

D. -
7
9

sin(a+

π
6
)=sin[
π
2
-(
π
3
-α)]=cos(
π
3
-α)=cos(α-
π
3
)=
1
3

则cos(2α-
3
)=2cos2(α−
π
3
)
-1=2×(
1
3
)
2
-1=-
7
9

故选D