Am I correct here with regards to the trig identity that was used?

Question

I am doing a physics problem where I am presented with the equation

$$\lvert \nu \rvert [\cos(\theta_1) + \cos(\theta_2)]/\lambda = 2 \lvert \nu \rvert [\cos(\theta_1 + \theta_2)/2 \cos(\theta_1 - \theta_2)/2]/\lambda$$

The way this equation is formatted makes it a bit confusing, but I think the trigonometric identity that is used here is

$$\cos(A) + \cos(B) = -2 \cos \left( \dfrac{A + B}{2} \right) \cos \left( \dfrac{A - B}{2} \right)$$

Am I correct here with regards to the trig identity that was used?

Answer 1

No the identity which you have written is wrong. The correct identity is for

$$\cos(A) + \cos(B) = 2\cos\dfrac{A+B}{2} \cos\dfrac{A-B}{2}$$