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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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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

4 Answers 4

up vote 14 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 $d$, where $d^2 = h^2 + z^2$, and $h^2 = x^2 + y^2$. So

$d^2 = (x^2 + y^2) + z^2$, and therefore $d = \sqrt{x^2 + y^2 + z^2}$

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The distance between two points in three dimensions is given by:
Given two points: point $a = (x_0, y_0, z_0)$; point $b = (x_1, y_1, z_1)$
The distance is (in units):
$$d = \sqrt{(x_1-x_0)^2 + (y_1-y_0)^2 + (z_1 - z_0)^2}$$
For your given points: point $a = (0,0,0)$; point $b = (1,2,3)$
Using substitution:
$$d = \sqrt{(1-0)^2+(2-0)^2+(3-0)^2}$$
$$d = \sqrt{(1 + 4 + 9)}$$
$$d = \sqrt{(14)}$$
$$d = 3.7$$
Note: if one point, point a, is the origin $$(0,0,0)$$ then the equation reduces to d = $\sqrt{(x^2 + y^2 + z^2)}$

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For some basic information about writing math at this site see e.g. here, here, here and here. – Chantry Cargill Dec 15 '14 at 20:40
Does d equal exactly 3.7? I get 3.7416573867739413 doing the calculation in Python. – ThorSummoner Feb 9 at 7:15

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