Nowadays, I realized that I'm interested in trigonometry and analytic trigonometry, which would be hard for my level. However, being curious makes whole things possible. Therefore, I'd like to get your book advices regarding trigonometric equations as well as multivariable trigonometric equations. For instance,
$$\cos (x)\cos (y)-\cos (x+y) = \dfrac{3}{2} \tag {1}$$
This question belong to pre-university calculus. As far as I can see, there are two variables in this equation. Thus, I couldn't find any similar equation on other trigonometric books I have.
How does the proof look like?
– jomegaA May 18 '20 at 09:52