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Math Help - Finding Max/Mins of a Function

  1. #1
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    Finding Max/Mins of a Function

    y = 2cosx + cos(2x)

    I guess I'm having trouble with the trig on this (i suck at trig).

    The 1st derivative would be -2sinx -2sin2x ... then I try to set that equal to zero and ... yeah. =/ I think sin2x = 2(cosx)(sinx) but then I don't know how to solve for that equal to zero. Any help would be appreciated. Thank you guys.
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  2. #2
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    Quote Originally Posted by drain View Post
    y = 2cosx + cos(2x)

    I guess I'm having trouble with the trig on this (i suck at trig).

    .
    I assume you want to maximize this on [0,2pi].

    y'= -2sin x - 2 sin 2x

    Thus,

    -2sin x - 2 sin 2x =0
    Divide by -2,
    sin x + sin 2x = 0
    Double angle identity,
    sin x + 2 sin x cos x =0
    Thus,
    sin x (1+2 cos x) =0
    Thus,
    sin x =0 or (1+2cos x) =0 on [0,2pi]
    Now find its zeros
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  3. #3
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    Thanks alot bro!
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