Hello TeriyakiDonnQ...(1)So this means, for example, that:
Now if you divide both sides of (1) by 2, you'll get:
... and so on.
Can you see, then, that if , this will give:
With the formula for , there's more than one possibility. You can start with:
but then, if you use the identity (which I'm sure you know), you can write, alternatively:or:
Which one of these three you use depends on the circumstances, but they are all correct.
You are given the expression . Well, that's just the RHS of the second of these three cosine formulae with . (If you simply didn't spot this, that's because you haven't had enough practice yet!)
So, using the second formula with , we get:Grandad