trigonometry question (high school level)

Question

Why does $\cos(50+60) = \cos(115)$?

I understand that $\cos(x+y) = \cos(x)\cos(y) - \sin(x)\sin(y)$,

but I dont understand how to apply this formula in this question or if I use another method to figure it out.

Thanks

Answer 1

It doesn't. Quite simply put, $\cos(50 + 60) \ne \cos(115)$ (arguments in degrees). You're running into difficulty because you're trying to prove something that isn't correct.