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Math Help - Solving for theta, a little rusty with trig identities

  1. #1
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    renton
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    Solving for theta, a little rusty with trig identities

    So it's been a while since I've needed to use trig identities to solve problems, so when I came across this one I found myself stuck pretty quickly.
    20 * cos^2(theta) = 35 * sin(theta) * cos(theta) + 3.0816

    what I tried so far was to change the sin and cos on the right side to 17.5 sin(2 theta)
    then I tried to change the cos^2 to 1 - sin^2
    and from there I thought I could maybe turn it into a quadratic, but with the second sin having 2 theta on the inside, wasn't really sure how to continue on this one.

    Any thoughts?

    thanks,

    Nick
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  2. #2
    Senior Member
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    Nov 2011
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    Crna Gora
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    Re: Solving for theta, a little rusty with trig identities

    \begin{array}{rcl}\sin \theta \cdot \cos \theta  & = & \frac{\sin 2\theta}{2} \\\cos^2 \theta & = & \frac{1+\cos 2\theta}{2} \\\sin^2 2\theta + \cos^2 2\theta & = & 1\end{array}
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