Question

求转换公式

Answer 1

sin(π/6 -α)

=cos[π/2 - (π/6 - α)]

=cos(π/3 + α)

=1/3

那么,

cos(2π/3 + 2α)

=cos[2(π/3 + α)]

=2*[cos(π/3 + α)]^2 - 1

=2*(1/3)^2 - 1

=2/9 - 1

= - 7/9

因此,答案应该是 A