Hello transgalactic Originally Posted by

**transgalactic** When I learnt these (about 200 years ago) I was recommended to use words, not symbols. After all, that's the way most of us learn our tables. It's a question of establishing a rhythm in your head. I've always recommended my students to learn them this way.

It goes like this:Sine plus sine is twice sine half-sum, cosine half-difference.

The next one swaps the sine and cosine around on the RHS:

Sine minus sine is twice cos half-sum, sine half-difference.

The next is the line of all cosines:

Cos plus cos is twice cos half-sum, cos half-difference.

The last one you remember, simply because it's different from all the rest; plus, being cosines it's the wrong way round. (Cosine is always awkward - when you differentiate it, for instance, you get a minus sine - pun intended.)

Cos minus cos is twice sine half-sum, sine half-difference *reversed*.

Just repeat these words over and over - you'll end up remembering them, just like learning a poem. Take my advice - don't try to learn them by writing them down using symbols and letters.

Grandad