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Math Help - A few questions, that I don't understand.

  1. #1
    Jagtogg
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    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.
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  2. #2
    Flow Master
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    Quote Originally Posted by Jagtogg View Post
    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 \cos (2 \theta) = 2 \cos^2 \theta - 1.
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