# What is the average distance between two random points inside a circle?

Assume you have a circle with some radius r. What is the average distance between two random points inside the circle?

(Edit: This is different from this already answered question, because here the points are inside the circle area, not on the circle circumference.)

• Do you mean the circle interior, or the circumference? – leonbloy Jun 3 '16 at 13:16
• I mean the interior – Kent Munthe Caspersen Jun 3 '16 at 13:18
• I don't think this is a duplicate of that question because that appears to be about points on the circle whereas this is about points in the circle. – Ian Jun 3 '16 at 13:50

## 1 Answer

Sketch. Let $S$ be the distance between the points, and let $X$ be the distance of the first point from the center of the circle.

Then compute:

• $F_{S|X}(s|x)=P(S \ge s | x)$

• $F_S(s)=P(S \ge s ) = \int F_{S|X}(s|x) \, f_X(x) \, dx$

• $E(S) = \int_0^\infty (1-F_S(s)) ds$

• Thanks, I think this is a good approach – Kent Munthe Caspersen Jun 3 '16 at 13:52