# A few questions, that I don't understand.

• Mar 21st 2008, 11:19 PM
Jagtogg
A few questions, that I don't understand.
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.
• Mar 22nd 2008, 03:44 AM
mr fantastic
Quote:

Originally Posted by Jagtogg
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.

For the first and third, first try reading this.

For the second, note that one version of the double angle formula for cosine is $\displaystyle \cos (2 \theta) = 2 \cos^2 \theta - 1$.