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Math Help - Completing the square

  1. #1
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    Completing the square

    Hi,

    Please help me understand an exam question I had last semester. I know how to find the solution if I could use b^2 = 4ac but I wasn't allowed to use that method and with completing the square, I would normally put the equation in unity, but it already is so I can miss that step, then take half the coefficients of the b term, move the constant to the other side of the equation, take half the coefficient of b and put it in a bracket, expand the bracket out to see what I need to add or subtract from the equation and bob should be my uncle, but I cant find the solution!

    Right the questions was:

    (i) By completing the square, find in terms of k the roots of the equation  x^2 + 2kx -7 = 0

    Any help would be greatly appreciated!

    Thanks,

    Kris
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  2. #2
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    Re: Completing the square

    Quote Originally Posted by Krislton View Post
    Right the questions was:
    (i) By completing the square, find in terms of k the roots of the equation  x^2 + 2kx -7 = 0

    That can written as (x+k)^2=k^2+7.
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  3. #3
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    Re: Completing the square

    Do you know how to multiply polynomials? In particular do you know that (x+ k)^2= x^2+ 2kx+ k^2? If so compare that with the your expression:
    x^2+ 2kx+ k^2 and x^2+ 2kx- 7 You need to get k^2 and get rid of -7. You can do that by adding [itex]k^2[/itex] and 7 to both sides of the equation.
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