Question

cosa=-3/5,a∈(π/2,π),则sin2a,cos2a的值是

Answer 1

sin2α＝2sinαcosα

cos2α＝cos^2(α)－sin^2(α)＝2cos^2(α)－1＝1－2sin^2(α)

所以sin2a=-24/25

cos2α=-7/25