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Almost half of them are trivial.
Then you're left with 6 sections, which are the less obvious one, half and double angle, sum and substraction, product, sum-to-product.
But they usually have roughly the same shape within a section. I'd write down them down, with one of them (usually for the sin function which is more common) at the top slightly bigger.
There are geometric proofs for each, whicn may or may not aid to memory (they're not exactly intuitive, but to associate a geometric shape to a formula).
Keep them for reference and you'll remember them as you see them appear in texts. The main issue for me is that when I see them, I feel they look arbitrary so when they show up it looks like sleight of hand. That's what the geometric proof is there for.
