已知sinα=23,则cos(3π−2α)等于( ) A.−53 B.19 C.-19 D.53
问题描述:
已知sinα=
,则cos(3π−2α)等于( )2 3
A. −
5
3
B.
1 9
C. -
1 9
D.
5
3
答
∵sinα=
,2 3
∴cos(3π-2α)=cos[2π+(π-2α)]
=cos(π-2α)=-cos2α=-(1-2sin2α)
=-1+2×
=-4 9
.1 9
故选C