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Math Help - What am I doing wrong in solving?

  1. #1
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    What am I doing wrong in solving?

    I'm back

    Alright, the answer should be -126 and 124. I am able to get 124 but not -126. Here is what I'm doing:

    (3-(x+1)^{2/3})/2 = -11

    3-(x+1)^{2/3} = -22

    -(x+1)^2/3 = -25

    (x+1)^{2/3} = 25

    x+1 = 125

    x = 124

    I'm only getting one answer. Where is the -126 coming from?
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  2. #2
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    Is there a plus/minus from a root you are not taking?
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  3. #3
    A Plied Mathematician
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    You should be a little more careful with your steps there. You have two mistakes which cancel each other out (line 2 to 3, and then line 3 to 4). Treat the problem like this:

    ((x+1)^{1/3})^{2} = 25. What does that suggest?
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  4. #4
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    Quote Originally Posted by Kieth89 View Post
    I'm back

    Alright, the answer should be -126 and 124. I am able to get 124 but not -126. Here is what I'm doing:

    (3-(x+1)^{2/3})/2 = -11

    3-(x+1)^{2/3} = -22

    -(x+1)^2/3 = -25

    (x+1)^{2/3} = 25

    x+1 = 125

    x = 124

    I'm only getting one answer. Where is the -126 coming from?
    Your second last line is wrong. It should be x + 1 = \pm 125 since you have taken the square root of both sides.
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  5. #5
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    Oh, I see now. Those get me every single time. I really need to remember the + and - deal whenever I'm solving with even roots.. Thanks for the help
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  6. #6
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    Quote Originally Posted by Kieth89 View Post
    I don't think so... it's not an even root.
    *Ahem* What do you think the power 2/3 means? To undo this power and hence get x + 1 you have cubed and then square rooted.
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