Question

若sin(α＋π/6)=1/3,则cos(2π/3－2α)的值为

Answer 1

cos(2π/3-2α)

=-cos[π-(2π/3-2α)]

=-cos(π/3+2α)

=-cos[2(π/6+α)]

=-{1-2[sin(π/6+α)]^2}

=-(1-2/9)

=-7/9