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Math Help - unique solution

  1. #1
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    unique solution

    For what values of k equation |x+1|-|x-1|=kx+1 have unique solution?
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  2. #2
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    Quote Originally Posted by Garas View Post
    For what values of k equation |x+1|-|x-1|=kx+1 have unique solution?
    Since absolute value has two separate formulas depending on whether the number inside the absolute value is positive or negative. That means you need to look at x+1< 0 (x< -1), x+1> 0 (x> -1), x-1< 0 (x< 1), and x-1> 0 (x> 1).

    That means we need to look at three intervals: x< -1, -1< x< 1, and x> 1.

    If x< -1, both x+1< 0 and x-1< 0 so the equation becomes -(x+1)-(-(x-1))= -2= 2k+1 which has no solution for x if x is not -3/2 but an infinite number of solutions if k= 3/2 (since then the equation reduces to -2= -2 which is true for all x).

    If -1< x< 1, x+1> 0 but x-1< 0 so the equation becomes (x+1)-(-(x-1))= 2x= 2k+1 which has a single solution for x no matter what k is.

    If x> 1, both x+1>0 and x-1> 0 so the equation becomes (x+1)- (x-1)= 2= 2k+1 which has no solution for x is k is not 1/2 but has an infinite number of solutions if k= 1/2 (because then the equation reduces to 2= 2 which is true for all x).
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  3. #3
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    What happened with x in -(x+1)-(-(x-1))= -2= 2k+1. Equation is |x+1|-|x-1|=kx+1. I don't understand that part.
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  4. #4
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    You said that there is infinite number of solutions if k=3/2 but i find only one solution and that is -2. abs&#40;x&#43;1&#41;-abs&#40;x-1&#41;&#61;&#40;3&#47;2&#41;x&#43;1 - Wolfram|Alpha
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  5. #5
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    Quote Originally Posted by Garas View Post
    What happened with x in -(x+1)-(-(x-1))= -2= 2k+1. Equation is |x+1|-|x-1|=kx+1. I don't understand that part.
    Misread again. See below.
    Last edited by HallsofIvy; June 29th 2010 at 10:06 AM.
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  6. #6
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    Quote Originally Posted by Garas View Post
    What happened with x in -(x+1)-(-(x-1))= -2= 2k+1. Equation is |x+1|-|x-1|=kx+1. I don't understand that part.
    My fault, I misread the equation! I accidently replaced the "x" with "2".

    For x< -1, the left side, as I said, reduces to -2 so the equation becomes -2= kx+ 1. Now subtract 1 from each side to get kx=-3 and divide by x to get x= -3/k, as long as k is not 0. If k= 0, this gives no solution.

    For -1< x< 1, the left side reduces to 2x so the equation becomes 2x= kx+ 1. subrtract kx from both sides to get 2x-kx= (2- k)x= 1 and then x= 1/(2- k) as long as k is not 2. If k= 2, this gives no solution.

    For x> 1, the left side reduces to 2 so the equation becomes 2=kx+ 1. subtract 1 from both sides to get kx= 1 and divide by x to get x= 1/k, as long as k is not 0. If k= 0, ths gives no solution.

    Now, what must k be so that only one of those gives a solution?
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  7. #7
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    I solved that on the same way and it wasn't correct. The right solution is: k goes from infinity to 0 and from 1 to infinity. It's not so easy as you might thought on beginning.
    Last edited by Garas; June 29th 2010 at 02:18 PM.
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