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Math Help - 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 \cos 45^\circ and \cos 30^\circ. From the identity \cos^2\alpha+\sin^2\alpha=1, you also know \sin 45^\circ and \sin 30^\circ. Maybe 75^\circ can be represented using 45^\circ and 30^\circ so that you can compute \cos 75^\circ using the info above?
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