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savvers

  • 2 years ago

-4sqrt(x+9)=20 can anyone help?

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  1. geoffb
    • 2 years ago
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    \[-4 \sqrt{(x+9)} = 20\] Like this? At what part are you getting stuck?

  2. savvers
    • 2 years ago
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    well i get x=16 but when i try to see if its true i get -20=20

  3. savvers
    • 2 years ago
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    and yes like that

  4. geoffb
    • 2 years ago
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    Yeah, with that -4 outside the square root, I think it's undefined.

  5. savvers
    • 2 years ago
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    yeah your probably right. extraneous solution?

  6. geoffb
    • 2 years ago
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    If we defined \(y\) as \(\sqrt{(x+9)}\), we would get \(-4y = 20\), with \(y = -5\), and there is no way \(\sqrt{(x+9)} = -5\)

  7. savvers
    • 2 years ago
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    hmmm i dont know. but when we divide -4 we get \[\sqrt{x+9}=-5\]

  8. geoffb
    • 2 years ago
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    \(\large{\sqrt{-x} = i \sqrt{x}}\) right?

  9. savvers
    • 2 years ago
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    yeah

  10. geoffb
    • 2 years ago
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    Okay, can we use that somehow?

  11. geoffb
    • 2 years ago
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    Oh wait... We can square both sides, right?

  12. savvers
    • 2 years ago
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    hmmm yeah

  13. savvers
    • 2 years ago
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    and we get x+9=25 because -5x-5=25

  14. geoffb
    • 2 years ago
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    Right, so you were right with 16; it is an extraneous root.

  15. savvers
    • 2 years ago
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    yeah it looks like that then and thanks man

  16. geoffb
    • 2 years ago
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    Hey, no problem. :)

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