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Math Help - Checking a Root of a Cubic

  1. #1
    Newbie danthegreat's Avatar
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    Checking a Root of a Cubic

    So I have this math problem...

    is -6 a root of....
    x^3-5x^2-3x-18=0? yes or no?

    i plug this equation in my calculator and go to the table. It says the coordinates are (-6,0) which means when I plug in -6, it should equal 0... However, when I do the work, I end up getting -183...

    (-6)^3-5(-6)^2-3(-6)-18=0
    -216 -5 +36 +18 - 18 =0

    -216-5+36 = -183 @_@?

    am I doing something wrong?
    Last edited by danthegreat; September 7th 2011 at 06:22 AM.
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  2. #2
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    re: Checking a Root of a Cubic

    In your work, you added -5 to 36 instead of multiplying .....but anyway

    I get positive 6 as an answer:

    6^3 - 5(6^2)-3(6)-18=0

    216-180-36-18=0
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  3. #3
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    re: Checking a Root of a Cubic

    Quote Originally Posted by danthegreat View Post
    So I have this math problem...

    is -6 a root of....
    x^3-5x^2-3x-18=0? yes or no?

    i plug this equation in my calculator and go to the table. It says the coordinates are (-6,0) which means when I plug in -6, it should equal 0... However, when I do the work, I end up getting -183...

    (-6)^3-5(-6)^2-3(-6)-18=0
    -216 -5 +36 +18 - 18 =0

    -216-5+36 = -183 @_@?
    Where did that come from?
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  4. #4
    Newbie danthegreat's Avatar
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    re: Checking a Root of a Cubic

    Oh wow...I am one huge dummy.

    I checked the coordinates (6,0), not -6. I rechecked and got (-6,-396) ??? Still doesn't make sense..

    Plato --

    (-6)^3-5(-6)^2-3(-6)-18=0
    -216 -5 +36 +18 - 18 =0

    -5 is brought down from above and then 36 is the result of -6 squared.

    EDIT: Figured out the problem stated above....Thanks! I forgot to multiply 5*36 and instead added them.
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