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Thread: [SOLVED] Trig Identities (short problem)

  1. #1
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    [SOLVED] Trig Identities (short problem)

    Hey guys, I need to find sin(3 $\displaystyle \pi $ /8)
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    Rhymes with Orange Chris L T521's Avatar
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    Quote Originally Posted by goliath View Post
    Hey guys, I need to find sin(3 $\displaystyle \pi $ /8)
    Note that $\displaystyle \sin\!\left(\tfrac{3\pi}{8}\right)=\sin\!\left(\tf rac{1}{2}\left(\tfrac{3\pi}{4}\right)\right)$.

    Now apply the half angle formula for sine with $\displaystyle \theta=\frac{3\pi}{4}$.

    Can you take it from here?
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  3. #3
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    Quote Originally Posted by goliath View Post
    Hey guys, I need to find sin(3 $\displaystyle \pi $ /8)
    Alternatively, remember that $\displaystyle \sin{\left(\frac{\pi}{2} - \theta\right)} = \cos{\theta}$.

    Can you see that $\displaystyle \frac{\pi}{2} - \frac{\pi}{8} = \frac{3\pi}{8}$?


    So that would mean $\displaystyle \sin{\frac{3\pi}{8}} = \cos{\frac{\pi}{8}}$.

    And you've been told what that is...
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