So hmm checking the picture below
you can pretty much just use the pythagorean theorem to get the missing adjacent side
once you have that, you just get the cosine since the cosine is just adjacent/hypotenuse
as far as the sum with θ, those angles would be in your Unit Circle surely
and just expand the sum identity with those values
you can pretty much just use the pythagorean theorem to get the missing adjacent side
once you have that, you just get the cosine since the cosine is just adjacent/hypotenuse
as far as the sum with θ, those angles would be in your Unit Circle surely
and just expand the sum identity with those values