Given that cos θ is equal to minus 3 / 5, and θ belongs to (π, 3 / 2 π), find the value of θ of COS 2

Given that cos θ is equal to minus 3 / 5, and θ belongs to (π, 3 / 2 π), find the value of θ of COS 2

Cos θ = - 3 / 5, θ∈ (π, 3 π / 2), then there is θ / 2 ∈ (π / 2, 3 π / 4), then there is cos (θ / 2) < 0
∵ cosθ=2cos^2(θ/2)-1,
There are cos (θ / 2) = - √ [(1 + cos θ) / 2]
=-√[（1-3/5）/2]
=-2√5/5.

What is cos (45-30?) equal to

cos（45°— 30°）
=cos(45)cos(30)+sin(45)sin(30)
=（√6+√2）/4

What is cos (45 ° - α) equal to? Is it cos α? Or what?
In fact, this problem is like this: how much is 2sin14 ° cos31 ° + sin17,

2sin14 ° cos31 ° + sin17 ° = 2sin14 (cos14cos17-sin14sin17) + sin17 = 2sin14cos17-2sin14sin14sin17 + sin17 = sin28cos17 + sin17 (1-2sin14sin14) = sin28cos17 + sin17cos28 = sin45 = radical 2 / 2