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Thread: Trig Identities

  1. #1
    Junior Member
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    Trig Identities

    Hello could someone please solve this question for me. I need to see the steps invloved as i am not really sure of what i am meant to be doing. Thankyou so somuch

    Question

    Using the identity sin^2theta + cos^2theta=1 and the identities for cos(A+B) find:
    cos 75degrees if cos 45degrees = .707, and cos 30degrees = 0.866
    Trig
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  2. #2
    MHF Contributor
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    You know $\displaystyle \cos 45^\circ$ and $\displaystyle \cos 30^\circ$. From the identity $\displaystyle \cos^2\alpha+\sin^2\alpha=1$, you also know $\displaystyle \sin 45^\circ$ and $\displaystyle \sin 30^\circ$. Maybe $\displaystyle 75^\circ$ can be represented using $\displaystyle 45^\circ$ and $\displaystyle 30^\circ$ so that you can compute $\displaystyle \cos 75^\circ$ using the info above?
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