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Derivative (max and min)

  1. Dec 13, 2009 #1
    Let f(x) = 5x^3 +4x + 8

    (i) find f'(x)

    answer--> 15x^2 + 4

    (ii) Show that f(x) is increasing on ([tex]-\infty,\infty[/tex])

    answer--> dont know how to prove it..

    (iii)Hence find the minimum and maximum value of f(x) on the closed
    interval [1,3]

    answer-->please guide me to start aswering this question..
  2. jcsd
  3. Dec 13, 2009 #2
    A function is increasing if the derivative is greater than zero. If the function is increasing then what are its maximum and minimum values in a closed interval?
  4. Dec 13, 2009 #3
    should i search for critical points first?
  5. Dec 13, 2009 #4
    If you like. But you can see directly from the expression of the derivative that it's never equal to zero.
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