42

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

How is this calculated?

Simon Verbeke
  • 627

4 Answers4

57

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 $$\sqrt{\displaystyle\sum_{i=1}^n (x_i -y_i)^2}$$

Aaron
  • 24,207
  • How can we prove the last formula? I find it difficult to imagine a n-dimensional space geometrically when $n > 3$... – user3019105 Apr 19 '18 at 10:30
  • 2
    @user3019105 We use induction, together with the fact that all the coordinate axes are orthogonal to each other. So the way the z-axis is orthogonal to the xy-plane (and hence to any line that has constant z-coordinate) generalizes. Or rather, that is how we define the geometry of higher dimensional space. – Aaron Apr 19 '18 at 15:31
  • Thank you, where can I find some theory and examples about this topic? – user3019105 Apr 19 '18 at 20:55
  • @user3019105 unfortunately, I don't know any references that talk about this in elementary terms, explaining why things are like they are. This is a basic example of a metric space (the Euclidean metric), and the metric is induced by a norm ($\ell^2$ norm), and that norm comes from an inner product, so this is an example in a lot of places, and I can give you references for generalizations, but I don't think they are what you are looking for. – Aaron Apr 19 '18 at 21:30
  • Whatever you have, if you could share it, I will be thankful and take a look! – user3019105 Apr 19 '18 at 21:33
  • 1
    A starting point might be this article on Wikipedia. – Aaron Apr 19 '18 at 21:37
17

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

Juan
  • 971
12

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

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

Phonon
  • 4,028
5

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

rayman may
  • 51