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Absolute value theorem that I can't convince myself of

  1. Sep 16, 2008 #1
    While reading my text, I came across an inequality that I couldn't convince myself of...

    For real numbers a,b: [tex]\left|a+b[/tex]|<= |a|+|b|. Is this something proven? Or is it an axiom or something?
  2. jcsd
  3. Sep 16, 2008 #2
  4. Sep 16, 2008 #3
    It's a pretty important inequality. I highly suggest you convince yourself of its truth :D
  5. Sep 17, 2008 #4
    To convince yourself of its truth consider what the effect of the signs of a and b have on the inequality.

    Case 1( a and b are positive):
    |a+b| = a + b = |a|+|b|

    Case 2 (a is positive and b is non-positive):
    Let b = -y then a and y are positive. If a-y is positive:
    [tex]|a+b|=|a-y| = a-y \leq a \leq |a| + |b|[/tex]
    If a-y is non-positive, then y-a is positive and:
    [tex]|a+b|=|a-y| = y-a \leq y = |b| \leq |a| + |b|[/tex]

    Case 3 (a and b are negative):
    Let a = -x, b = -y:
    |a+b| = |-(x+y)| = x+y = |a|+|b|
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