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Thread: polynomial absolute value

  1. #1
    Mar 2013

    polynomial absolute value

    Reading through this book that gives a problem, explains why a solution is wrong, but I don't understand it.

    The problem is, given x = 2, y= 4, evaluate:

    x+ 2y+ sq root of (x-2y)2

    It explains that the sq root of (x-2y)2 = (x-2y) only if x >= 2y. I understand that if 2y > x, you would have a negative number if you simply drop the square and the square root symbols.

    They explain that the sq root of (x-2y)2 is the |(x-2y)| in all cases.

    The expression they give after removing the square root and the square of (x-2y) is:

    x +2y +2y - x

    I understand where the (x+ 2y) comes from (original expression). What I don't understand is how to get from |(x-2y)| to (2y-x).

    In this case we know the value of x and y, but if we didn't, how would we obtain the absolute value of a polynomial? Simply changing the sign doesn't seem like it would work in all cases.
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  2. #2
    Senior Member
    Feb 2013
    Saudi Arabia

    Re: polynomial absolute value


    |x| = x if x> 0 and |x| = -x if x<0

    this is the definition of |x| your case (2y-x)=8-2=6>0 and (x-2y)<0 therefore |x-2y|=-(x-2y) = 2y-x

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