Let q1 and q2 be two points in the plane. Then the set of all points p with constant distance ratio from them is a line or circle.

Asked Jan 09 '13 at 23:43

Active Jul 05 '15 at 14:24

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I saw the statement in the question title online. It doesn't seem hard to show this is true simply by manipulating the expression $\frac{abs(p-q_1)}{abs(p-q_2)}=d$ (though I haven't done this), but there has to be some geometric explanation I'm missing. I can, geometrically, see why the set describes a line when $d=1$ (we construct a triangle with edges $p$, $q_1$, $q_2$ and look at the median), but why is it a circle otherwise?

edited Jul 05 '15 at 14:24

Harish Chandra Rajpoot

37,450

asked Jan 09 '13 at 23:43

ta33

1

This phenomenon is called Apollonius Circle. It has a nice projective geometry solution, and of course, a brute force coordinate geometry solution. – Calvin Lin Jan 09 '13 at 23:50

Let q1 and q2 be two points in the plane. Then the set of all points p with constant distance ratio from them is a line or circle.

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