How to remember the trigonometric identities I have a test tomorrow and I am having trouble remembering those pesky trigonometrical identities (such as $1-\cos x=2\sin^2(\frac{x}{2})$ )
Do you guys have any tips on how I can remember these?
Thanks :)

you should remember the following
$$
\sin(a\pm b)=\sin(a)\cos(b)\pm\cos(a)\sin(b)
$$
$$
\cos(a\pm b)=\cos a\cos b\mp\sin a\sin b
$$
$$
\cos^2x+\sin^2x=1
$$
from these (with $x=a=b$) you can get
$$
\cos(2x)=\cos^2x-\sin^2x=1-2\sin^2x=2\cos^2x-1
$$
and various other rearrangements.
this may not be helpful, but when I couldn't remember sine/cosine of a sum (ie before teaching) i would use
$$e^{i\theta}=\cos\theta+i\sin\theta$$ and
$$
e^{i(\theta+\phi)}=e^{i\theta}e^{i\phi}
$$
to find them (or check my memory)
