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Given three complex values (for example, $2i, 4, i+3$), how would you calculate the equation of the circle that contains those three points? I know it has something to do with the cross ratio of the three points and $z$ and the fact that the cross ratio is a real number, but I don't know what to do with that information.

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Do you know what a Möbius transformation is? If so, the inverse of the Möbius transformation given by the cross-ratio gives you a parametrization of the circle (up to one among $2i$, $4$, $i+3$ depending on your exact formula of the cross ratio) by the real numbers. –  t.b. May 15 '11 at 4:17
    
If you need to verify the result you get from Möbius, you can use the usual determinant for the Cartesian equation of a circle through three points (treating the complex plane as a Cartesian plane) and check that the parametrization you obtain (by replacing $x$ and $y$ with appropriate expressions) satisfies that Cartesian expression. –  J. M. May 15 '11 at 4:21
    
"the inverse of the Möbius transformation given by the cross-ratio" what do you mean by given by? –  Rebakah May 15 '11 at 4:25
    
The cross-ratio is a special Möbius transformation (the unique one that maps $z_1, z_2, z_3$ to $1,0,\infty$, say - again depending on your conventions - it might be $0,1,\infty$ as well). –  t.b. May 15 '11 at 4:31
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just take perpendicular bisectors of any two sides of the triangle..the point of intersection of two bisectors is the center..now find the distance between center and one of the vertices..it is the radius.. done. –  Dinesh May 15 '11 at 5:04

2 Answers 2

"Might as well flesh out" @Dinesh's comment.

This solution uses algebraic manipulations and consideration of the geometry of the figures. [A more concise solution might leverage vector formality.]

The solution is the equation in the form (x-h)^2+(y-k)^2=r^2, where we give the values of the 3 parameters, h, k, and r.

First, we convert to Cartezian coordinates:

2i -> (0,2)
4 -> (4,0)
i+3 -> (3,1)

1 -- To find the center of the circle (h,k), we need the equations of two lines that are perpendicular bisectors of any two of the sides of the implied triangle above. The intersection of these lines is the center of the circle.

2 -- A perpendicular bisector will have a slope, M, of -1/m, where m is the slope of the corresponding side of the triangle. We get the slope of a side, m, easily from the two given points for that side. We also can easily derive a point, P, on the perpendicular bisector: the midpoint of the side of the triangle. With a point and slope we essentially have the line equations we need.

3 -- To get the length of the radius, r, we compute the distance between the center of the circle and any of the points on the circle.

The perpendicular bisectors of the sides of a triangle intersect in the center of the circumscribing circle of that triangle. The proof is easy, as we note that every point along one of these perpendicular bisectors is equidistance from each of the two points that form the ends of the corresponding triangle side. Thus, the center of this circle (being the intersection of these perpendicular bisectors) has the property that it is equidistant from each of the 3 points, ie, it is a circumscribing circle. Note that given any 3 points, there is a single triangle implied by them (so we can use the perpendicular bisector argument to conclude that a single circle exists).

Now, for key calculations:

2:

m_1 = (0-1)/(4-3) = -1
m_2 = (2-1)/(0-3) = - 1/3
M_1 = -1/(-1) = 1
M_2 = -1/(-1/3) = 3
P_1 = ( (4+3)/2 , (0+1)/2 ) = (7/2,1/2)
P_2 = ( (0+3)/2 , (2+1)/2 ) = (3/2,3/2)

1:

EQN_1

P_1y = (M_1)(P_1x) + B_1
1/2 = (1)(7/2) + B_1
B_1 = -3

Y = X - 3

EQN_2

P_2y = (M_2)(P_2x) + B_2
3/2 = (3)(3/2) + B_2
B_2 = -3

Y = 3X - 3

Intersection

X - 3 = 3X - 3
X = 0

Y = (0) - 3
Y = -3

Thus, h = 0 and k = -3

3:

We can calculate distance from (h,k) to (0,2)

r = sqrt [ (0 - 0)^2 + (-3 - 2)^2 ]
r = 5

More specifically, the equation of the circle is:

(x)^2 + (y + 3)^2 = 25
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Might as well flesh out Theo's comments a bit...

If you have four points $z_i,\quad i=1\dots4$, in the following configuration:

cross-ratio

the cross ratio of these four points is

$$\Delta=\frac{z_1-z_3}{z_1-z_4}\left(\frac{z_2-z_3}{z_2-z_4}\right)^{-1}$$

If $\Delta$ is real, this means that

$$\arg\left(\frac{z_1-z_3}{z_1-z_4}\right)-\arg\left(\frac{z_2-z_3}{z_2-z_4}\right)=0$$

Interpreted geometrically, the angles $\angle z_3 z_1 z_4$ and $\angle z_3 z_2 z_4$ are congruent, and must thus be inscribed angles in a circle; i.e. all four points are concyclic.

If we let $z_4$ be a variable point $z$ tracing the circle and $\Delta$ be a varying parameter, we obtain an equation for the circle through $z_1,z_2,z_3$:

$$\Delta=\frac{z_1-z_3}{z_1-z}\left(\frac{z_2-z_3}{z_2-z}\right)^{-1}$$

Solving for $z=x+iy$ gives

$$z=\frac{z_2(z_1-z_3)-z_1(z_2-z_3)\Delta}{z_1-z_3-(z_2-z_3)\Delta}$$

which as Theo says is a Möbius transformation. Taking real and imaginary parts and eliminating $\Delta$ should yield a Cartesian equation. (Obtaining the center and radius of this circle is a bit messy and is left as an exercise in algebraic manipulation.)

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Nice, thank you! The $x$-coordinate of the center can be found in the link I provided in my last comment above. This shows that the Cartesian equation must be even worse in terms of ugliness. But it may well be that the points in the OP's question are chosen in such a way that the equations work out nicely - I haven't bothered to check. –  t.b. May 15 '11 at 5:57
    
@Theo: It all checks out algebraically, but I had to use Mathematica for verification since I'm trying to conserve paper here... XD –  J. M. May 15 '11 at 6:05
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As a bonus: for a circle in Möbius form $z=\frac{a+bt}{c+dt}$, the center is given by the expression $$p=\frac{a\bar{d}-b\bar{c}}{c\bar{d}-d\bar{c}}$$ and the radius is given by $$r^2=|p|^2-\frac{a\bar{b}-b\bar{a}}{c\bar{d}-d\bar{c}}$$ ; rather compact in complex form, but hellish-looking when you go Cartesian... –  J. M. May 15 '11 at 6:20
    
Yes, thank you! This is the thing you get when verifying that Möbius transformations preserve circles. I can remember using half of Brazil's wood supply when verifying this stuff while going through Ahlfors. –  t.b. May 15 '11 at 6:23

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