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Math Help - Completing the square

  1. #1
    Senior Member Mukilab's Avatar
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    Completing the square

    Confused how you get from step 1 to step 2


    I don't understand how you get the -7 from x^2-2x
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  2. #2
    A Plied Mathematician
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    This looks like you picked up the algebra in the middle of a problem. Could you please state the original problem? Thanks!
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  3. #3
    Senior Member Mukilab's Avatar
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    Solve x2 = 2x + 7 (a similar equation to the earlier example)

    First rearrange to get: x2 - 2x − 7 = 0

    Rough Working for the solution:
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  4. #4
    A Plied Mathematician
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    Ok. That makes more sense now. Your first step is fine, although I would probably leave the 7 on the RHS. Different people complete the square in different ways.

    x^{2}-2x=7. I pulled the 2x over to the LHS.
    x^{2}-2x+1=7+1. I added the square of half the coefficient of the x term to both sides. Then you get
    (x-1)^{2}=8. From here, it looks as though you've solved the rest of the problem.
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  5. #5
    Senior Member Mukilab's Avatar
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    Quote Originally Posted by Ackbeet View Post
    Ok. That makes more sense now. Your first step is fine, although I would probably leave the 7 on the RHS. Different people complete the square in different ways.

    x^{2}-2x=7. I pulled the 2x over to the LHS.
    x^{2}-2x+1=7+1. I added the square of half the coefficient of the x term to both sides. Then you get
    (x-1)^{2}=8. From here, it looks as though you've solved the rest of the problem.
    thanks, you're explanation is much better
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  6. #6
    A Plied Mathematician
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    You're welcome. Have fun!
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