# Math Help - how to remmember these trigonometric formulas

1. ## how to remmember these trigonometric formulas

Table of Trigonometric Identities
sum to product formulas part
i am looking for a logical way of remmembering them

2. Originally Posted by transgalactic
Table of Trigonometric Identities
sum to product formulas part
i am looking for a logical way of remmembering them
I'm not sure there's many shortcuts for this. It's just a case of strapping down and memorising them. Write them out a few hundred times. Derive them a few hundred times as knowing where they come from can help. Write them on paper and pin them to the ceiling above your bed or whatever.

Personally, I find it best just to remember quick versions of the derivations, that way I don't have to memorising anything except the original equations.

3. Hello transgalactic
Originally Posted by transgalactic
Table of Trigonometric Identities
sum to product formulas part
i am looking for a logical way of remmembering them
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.

4. what rithm to use

like hiphop/ or like acid trance or like country rithm

it doesnt rihm at all

"Sine plus sine is twice sine half-sum, cosine half-difference"

5. its not going to be like this
$
a_{-n} = \frac{1}{2\pi}\int_{-\pi}^{\pi} \! f(t)e^{-i(-n)t} \, dt
$

you forgot to put a line above the f(x)