If cos α = 1 / 3 and α belongs to (π, 2 π), then cos π / 2 is equal to

If cos α = 1 / 3 and α belongs to (π, 2 π), then cos π / 2 is equal to

Cos π / 2 = 0 is independent of angle α
Let's find COA α / 2
∵cosα=1/3
According to the double angle formula:
cosα=2(cosα/2)^2-1
∴(cosα/2)^2=(1+cosα)/2=2/3
∵ α belongs to (π, 2 π)
That α / 2 belongs to (π / 2, π)
∴cosα/2

The value of COS (2 / 3 π) is equal to?

The answer is 1 / 2

Cos (2 π / 3 - α) energy is equal to cos α?
There is a question whose answer says - cos (2 π / 3 + 2 α) = cos 2 α, so I want to ask whether it is possible to do this? There is no such formula in the induction formula

It can't be used directly, only α is a special angle
- cos (2 π / 3 + 2 α) = Cos2 α reduction
cos(2π/3+2α)=-cos2α
-1/2cos2a-√3/2sin2a=-cos2a
1/2cos2a-√3/2sin2a=0
sin(π/6-2a)=0
π/6-2a=kπ