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
,则cos(2a-1 3
)的值是( )2π 3
A.
7 9
B.
1 3
C. -
1 3
D. -
7 9
答
sin(a+
)=sin[π 6
-(π 2
-α)]=cos(π 3
-α)=cos(α-π 3
)=π 3
,1 3
则cos(2α-
)=2cos2(α−2π 3
)-1=2×(π 3
)2-1=-1 3
7 9
故选D