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A critical numbers problem

  1. Mar 29, 2007 #1
    find the critical number of the function

    f(X)= x^3 + 3x^2-24x

    f'(X)= 3x^2 + 6x -24

    what do i do, next i found the derivative of the function
     
  2. jcsd
  3. Mar 29, 2007 #2

    arildno

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    Well, perhaps you should find out either where the derivative equals 23.67, -17 or perhaps some more important value, say 0.
     
  4. Mar 29, 2007 #3
    ok so its supposed to equal zero as the definition states for critical numbers,

    so i just plug in any numbers in the x's
     
  5. Mar 29, 2007 #4

    arildno

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    Eeh??
    Do you know what an equation is, and how to solve it??
     
  6. Mar 29, 2007 #5
    i think ido, can u show me an example
     
  7. Mar 29, 2007 #6
    yes i know what an equation is, but how do i solve it
     
  8. Mar 29, 2007 #7

    hage567

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    Can you name what type of equation that is?
     
  9. Mar 29, 2007 #8
    well im on the section maximum and minum values, it doesnt say what kind of equation it is, id say its a critical number equation
     
  10. Mar 29, 2007 #9

    hage567

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    :confused: :confused:

    Just look at its form. It's a QUADRATIC equation! Have you not learned about those before? There's a formula for finding the roots of a quadratic equation. If you don't know it, look it up.
     
  11. Mar 29, 2007 #10
    ok, so when i take the derivative, i just factor it out
     
  12. Mar 29, 2007 #11

    hage567

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    What?? I don't understand what you mean by that.
     
  13. Mar 29, 2007 #12
    well ive learned about quadratic equations but i forgot what it was called,
    well when i factored it out, i got 3(x^2+2x-8) then i factored it out

    3(x+2)(x-4) ok thats easy, i forgot about that algebra stuff and trig and geo, like my calculus teacher said, it's not the calculus its all that old stuff that will get ya
     
  14. Mar 29, 2007 #13
    i mean 3 (x-2) (x+4)
     
  15. Mar 29, 2007 #14

    hage567

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    OK that's better I was just going to point that mistake out.
     
  16. Mar 29, 2007 #15
    yea hey thanks for reminding me about that,
     
  17. Mar 29, 2007 #16

    hage567

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    No problem. But I would recommend looking up the quadratic formula and remembering it. It works for those times when factoring isn't obvious.
     
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