Prove using a trigonometric identity.

Question

Prove the following using a trigonometric identity:

 

cos(2θ)=cos(θ)

Answer 1

If you want a solution.

 

2cos² θ - 1 = cos θ

2cos² θ - cos θ - 1 = 0

(2cos θ + 1)(cos θ - 1) = 0

 

2cos θ = -1

or

cos θ = 1

 

Can you solve and answer?

Answer 2

There is no possible proof as this isn't true for all theta. Did you mean you wanted to solve the equation for certain values of theta?