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Question : Prove $|d(x, z) − d(y, z)|$ is less than or equal to $d(x, y)$.

I know I have to use the triangle inequality but I'm just not sure how to apply it with a negative $d(y,x)$.

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$d(x,z) \leq d(x,y) + d(y,z)$ gives $d(x,z) - d(y,z) \leq d(x,y)$. Similarly, $d(y,z) \leq d(y,x) + d(x,z)$ gives $d(y,z) - d(x,z) \leq d(y,x) = d(x,y)$, so $\pm(d(x,z) - d(x,y)) \leq d(x,y)$. Now think about what the value of $|d(x,z)-d(y,z)|$ is by definition. –  t.b. Apr 10 '12 at 9:23
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Answered this not long ago. Probably a multiplicate. –  Did Apr 10 '12 at 15:24
    
And proofwiki.org/wiki/Reverse_Triangle_Inequality. –  Did Apr 10 '12 at 15:32
    
I have edited the question to make the question inline. Please post questions in the body and a big hint about the question in the title. Regards, –  user21436 Apr 11 '12 at 0:43

2 Answers 2

up vote 2 down vote accepted

The claim is invariant under exchange of $x$ and $y$. Thus without loss of generality we can assume $d(x,z)\ge d(y,z)$. Then $|d(x,z)-d(y,z)|=d(x,z)-d(y,z)$, which is $\le d(x,y)$ by the triangle inequality.

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First note that $|a-b|\leq |a|+|b|$\ Now, $|d(x,z)-d(y,z)|\leq |d(x,z)|+|d(y,z)|=d(x,z)+d(y,z)\leq d(x,y) (\because \text{triangle inequality}).$

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This doesn't work this way. The triangle inequality reads $d(x,y) \leq d(x,z) + d(y,z)$, not the other way around. –  t.b. Apr 10 '12 at 17:33

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