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gabycc12

find the critical numbers and the open intervals on which the function is increasing or decreasing. y=3x^3+12x^2+15x

  • one year ago
  • one year ago

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  1. myininaya
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    To find critical numbers for a polynomial you need to find f'(x) And set f'(x)=0 And solve for x These solutions to f'(x) will be your critical numbers

    • one year ago
  2. ParthKohli
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    lol. It took me time to realize that "critical numbers" just means "critical points". :P

    • one year ago
  3. myininaya
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    Do you know how to find y'?

    • one year ago
  4. gabycc12
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    f'(x)= 9x^2+24x+5 correct??

    • one year ago
  5. myininaya
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    close

    • one year ago
  6. myininaya
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    (15x)'=15

    • one year ago
  7. gabycc12
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    oh okay now what do i do after this ? f'(x)= 9x^2+24x+15

    • one year ago
  8. myininaya
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    Please refer to my instructions. Set f'(x)=0 and solve for x :)

    • one year ago
  9. gabycc12
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    i cant seem to be able to solve for x i got 3x(3x+8)=15

    • one year ago
  10. myininaya
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    This is a quadratic equation You may use the quadratic formula

    • one year ago
  11. gabycc12
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    yeah i did that and i got 8 and -8 ; but the answers in the book are -1 and -5/3

    • one year ago
  12. myininaya
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    So you have 9x^2+24x+15=0 correct?

    • one year ago
  13. myininaya
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    You could divide both sides by 3 since each term has a common factor 3 Giving you 3x^2+8x+5=0 Now you can do the quadratic formula but this is also pretty easy to factor

    • one year ago
  14. myininaya
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    \[ax^2+bx+c=0 => x=\frac{-b \pm \sqrt{b^2-4ac}}{2a}\] Did you use this formula?

    • one year ago
  15. gabycc12
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    yes i did

    • one year ago
  16. myininaya
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    Then you need to try again, because 8 and -8 is not what you should get.

    • one year ago
  17. myininaya
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    What is a=? What is b=? What is c=?

    • one year ago
  18. gabycc12
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    a=9 b=24 c=15 correct??

    • one year ago
  19. myininaya
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    Sure are you could have used the equation I provided you So let's plug these numbers in that you have

    • one year ago
  20. myininaya
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    \[x=\frac{-24 \pm \sqrt{24^2-4(9)(15)}}{2(9)}\]

    • one year ago
  21. myininaya
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    Use order of operations to evaluate what is inside the radical

    • one year ago
  22. gabycc12
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    ohhh okay i see what i did wrong i multiplied -24 by 6 instead of adding and subtracting. Thank you so much for your help!

    • one year ago
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