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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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    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,2012-04-11

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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.2012-04-10