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Find the values of $R$ and $\alpha$ in the identities below, given that $R>0$ and $\alpha$ is an acute angle.
I'm a bit confused by this task.
How should I start? I have $$ \cos(a+b) = \cos(a)\cos(b)-\sin(a)\sin(b). $$
If I square anything, I can use the trig identity $$ \sin^2(x) + \cos^2(x) =1. $$