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Old 09-06-2008, 10:15 PM
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Default Proof concerning absolute values and Triangle Inequality

Hey guys ...I haven't been online for six months. I need your help with the following question.


Prove that \mid \mid a \mid - \mid b \mid \mid \leq \mid a-b \mid

Thank You
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Old 09-06-2008, 10:21 PM
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\begin{aligned}   \left| a-b \right|^{2}&=(a-b)^{2} \\  & =a^{2}-2ab+b^{2} \\   &\ge \left| a \right|^{2}-2\left| a \right|\left| b \right|+\left| b \right|^{2} \\  & =\Big[ \left| a \right|-\left| b \right| \Big]^{2} \\   \therefore\quad \left| a-b \right|&\ge \Big| \left| a \right|-\left| b \right| \Big|.\quad\blacksquare \end{aligned}
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Old 09-06-2008, 10:27 PM
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Originally Posted by hercules View Post
Hey guys ...I haven't been online for six months. I need your help with the following question.


Prove that \mid \mid a \mid - \mid b \mid \mid \leq \mid a-b \mid

Thank You
I like Kriz's method! here's an alternate method:

|a| = |(a - b) + b| \le |a - b| + |b| (The \triangle-inequality)

Thus, we have |a - b| \ge |a| - |b|

similarly, we can start with |b| and deduce that |a - b| \ge |b| - |a| \implies -|a - b| \le |a| - |b|

putting them together we have -|a - b| \le |a| - |b| \le |a - b| \implies ||a| - |b|| \le |a - b| by the definition of absolute values
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Old 09-06-2008, 10:29 PM
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Originally Posted by Jhevon View Post

(The \triangle-inequality)
This can be proved in the same fashion as I did above.

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Old 09-06-2008, 10:36 PM
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This can be proved in the same fashion as I did above.

yes, i know. that's the kind of proof i have for it

i just used this method because he mentioned triangle inequality in the original post
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Old 09-06-2008, 10:38 PM
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Thank You both replies were really helpful.
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