For the first and third, first try reading this.
For the second, note that one version of the double angle formula for cosine is .
I'm learning how to use Trig Identities to complete simple problems. The only problem, is that I don't know how to do certain questions when the multiply or divide theta.... examples:
If cos theta = -4/5 for theta in Quadrant II, find sin theta/2.
If cos theta = -4/5 for theta in Quadrant II, find cos 2theta.
If sin theta = 5/13 for theta in Quadrant I, find cos theta/2.