1. Which formula do you use for the cosine rule? A formula that isn’t listed above?
2. Why might you stress the importance of number 2?
3. Is there a context in which you would consider teaching number 3? Is it a problem that this doesn’t appear in textbooks or formula books?
Dan Pearcy 
I think you’re missing a `^2` on the left-hand side of that third equation.
In the first one, I would permute the variables so that `c` is on the LHS. This avoids the double `c` in `b c cos` (which when typing could also be avoided by using `\cos` instead of `cos` but I’d still avoid this because the distinction is harder to make in handwriting).
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I prefer going visual with the connection to the geometry rule SAS.
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