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Math Help - Is this trigonometric identity question correct?

  1. #1
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    Is this trigonometric identity question correct?

    Given cosθ=-3/5 and π < θ < 3π/2, use an appropriate trigonometric identity to evaluate exactly.

    1) cos θ/2

    I got :

    √(1+cosθ/2) = √(1+(-3/5)/2) = √((2/5)/2) = √(5)/5

    I realize that they are referring to quadrant 3, I'm not really sure why that is relevant in the question. Could someone please explain that to me? And also if I have the right answer?

    Thank you very much!
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  2. #2
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    Re: Is this trigonometric identity question correct?

    Hey curt26.

    Just to confirm, are you using the double angle relationship (cos(2x) in terms of cos(x)) to answer your question?
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  3. #3
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    Re: Is this trigonometric identity question correct?

    Sorry for the late reply. I have limited internet access when I'm out of town working.

    No I was using the Half-angle identity...Is that not correct?

    Thank you
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  4. #4
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    Re: Is this trigonometric identity question correct?

    Quote Originally Posted by curt26 View Post
    Sorry for the late reply. I have limited internet access when I'm out of town working.
    No I was using the Half-angle identity...Is that not correct?
    From the given you know that \frac{\pi }{2} < \frac{\theta }{2} < \frac{{3\pi }}{4} so \cos \left( {\frac{\theta }{2}} \right) =  - \sqrt {\frac{{1 + \cos (\theta )}}{2}}
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  5. #5
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    Re: Is this trigonometric identity question correct?

    Yes thank you. That's the formula I used
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