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The distance between point and line independently on metric is defined by $$d(X, l) = \inf\{d(X, Y)|Y\in l\}.$$ I have troubles understandning how this infimum works.

Can someone please give me an explanation why it is defined like this and how it works with examples and explanations in $\mathbb{R}^2$ for euclidean distance $d_e((x_1, x_2), (y_1, y_2)) = \sqrt{(x_1 - y_1)^2 + (x_2 - y_2)^2}$, maximum distance $\max\{|x_1 - y_1|, |x_2 - y_2|\}$ and manhattan distance $|x_1 - y_1| + |x_2 - y_2|$?

I'm having really hard time understanding this.

Many thanks!

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  • $\begingroup$ The distance from a point to any object is defined as the closest distance to the object. If you take the distance from $X$ to each point $Y$ on the line, there will be a smallest distance. That's the one that defines the distance to the line. $\endgroup$ – John Douma Apr 12 '16 at 15:08
  • $\begingroup$ And so the $\inf$ assures, that the distance we are taking is the closest one? $\endgroup$ – Accelerate to the Infinity Apr 12 '16 at 15:36

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