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Imagine I want to determine the distance between points 0,0,0 and 1,2,3.

How is this calculated?

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1  
How would you do it in two dimensions? –  JavaMan Jun 1 '11 at 20:00
    
No idea. For some reason they don't learn us that at school… –  Simon Verbeke Jun 1 '11 at 20:12

3 Answers 3

up vote 6 down vote accepted

By using the the Pythagorean theorem twice, you can show that $d((0,0,0),(1,2,3))=\sqrt{\left(\sqrt{1^2+2^2}\right)^2+3^2}=\sqrt{1^2+2^2+3^2}$.

In general, if you have two points $(x_1, \ldots, x_n)$ and $(y_1, \ldots, y_n)$ in $\mathbb{R}^n$, you can use the Pythagorean theorem $n-1$ times to show that the distance between them is $$\sqrt{\displaystyle\sum_{i=1}^n (x_i -y_i)^2}$$

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It's Pythagorean theorem, just like with 2D space.

$||[0, 0, 0]-[1, 2, 3]|| = \sqrt{(0-1)^2+(0-2)^2+(0-3)^2} = \sqrt{1+4+9} = \sqrt{14}$

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Here is an illustration:

3d Pythagorean theorem illustration

You want to find $d = \sqrt{h^2 + z^2}$, where $h = \sqrt{x^2 + y^2}$. So

$d = \sqrt{\sqrt{x^2 + y^2}^2 + z^2} = \sqrt{x^2 + y^2 + z^2}$

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