I ran into this trig identity trying to do my physics hw:(adsbygoogle = window.adsbygoogle || []).push({});

a*cos(s)+b*cos(t)

I tried deriving it using an analogous approach to deriving the product-sum trig identity, but ran into problems. I was wondering if this trig identity exists, or if I should just try to find some clever way to solve the physics problem without the trig identity.

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# Problem with this trig identity

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