TITLE: How to memorize the trigonometric identities?
QUESTION [8 upvotes]: I am stuck trying to memorize the trig identities, and try as I may, I just can't get them to stick (especially the sum-product and product-sum formulas). I am concerned I won't be able to memorize them in time for my test, and I was wondering if there was a better way than rhote memorization.
Any suggestions?
Thanks.
EDIT: Passed the test, thanks to your good suggestions. Thank you all!

REPLY [3 votes]: The way that I used to memorize the sum of angle formulas is by memorizing the double angle formulas instead. For example
$$\sin 2\theta = 2\sin\theta\cos\theta = \sin\theta\cos\theta+\cos\theta\sin\theta$$
reminds me that
$$\sin(A+B) = \sin A\cos B + \cos A\sin B$$
similarly
$$\cos2\theta = \cos^2\theta-\sin^2\theta = \cos\theta\cos\theta - \sin\theta\sin\theta$$
reminds me that
$$\cos(A+B) = \cos A\cos B - \sin A\sin B$$