已知sin(π6−a)=13,则cos(2π3+2a)的值是( ) A.-79 B.-13 C.13 D.79
问题描述:
已知sin(
−a)=π 6
,则cos(1 3
+2a)的值是( )2π 3
A. -
7 9
B. -
1 3
C.
1 3
D.
7 9
答
cos(
+2α)=-cos(2π 3
-2α)π 3
=-cos[2(
−α)]π 6
=-[1-2sin2(
−α)]π 6
=-(1-
)=-2 9
7 9
故选A