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Math Help - Maximum and Minimum values

  1. #1
    Junior Member
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    Maximum and Minimum values

    Find the absolute max and absolute min values of f on the given interval.
    1. f(t) = 2 cost + sin 2t, interval [0, pie/2]
    2. f(t) = x - ln x , [1/2, 2]
    For #1, I figured out the f '(t) which is -sin t + cos 2... I don't know if this is correct. the question is how do I find the critical pts with this f '(t)? Same question for #2. The f '(t) that I figured out is 1/x... How do I find the critical pts? Thanks for your help.
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  2. #2
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    Critical values are points where the derivative is equal to zero or undefined, so:

    1) f(t) = 2 cost + sin 2t

    You messed up your derivative. The derivative on cos(t) is -sing(t). Since 2 is a constant it comes along for the ride. for sin(2t) you can use the chain rule:

    f '(t) = -2 sin(t) + cos(2t) (2)

    Now set this equal to zero:

    0 = -2sin(t) +2cos(2t)

    0 = -sin(t) + cos(2t)

    Using a trig identity we have:

    0 = -sin(t) + [1 - 2(sin(t))^2]
    0 = 2(sin(t))^2 + sin(t) -1
    0 = (2sin(t) -1)(sin(t) +1)

    0 = 2sin(t) - 1 ==> sin(t) = 1/2 <==> t = pi/6

    0 = sin(t)+1 ==> sin(t) = -1 which also does not happen in your interval, so we can ignore this point as well.

    So now you have to evaluate 0, pi/6 and pi/2 to find the absolute max and min.

    2. f(t) = x - ln x , [1/2, 2]

    f '(t) = 1 - (1/x)

    Setting this equal to zero:

    0 = 1- (1/x)

    (1/x) = 1

    (1) = x

    Now you evaluate 1/2, 1 and 2 to find the absolute max and minimum.
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