Conics Circle Problem

dagr8est

Junior Member
Joined
Nov 2, 2004
Messages
128
In the form (x-a)^2+(y-b)^2=r^2, write the equation of a circle which passes through A(0,-9), B(7, -2) and C(-5, -10). I can't figure out how to start this question.
 
The perpendicular bisector of AB is x+y=−5/2.
The perpendicular bisector of BC is 3x+y=−9.
Use those to find the center and radius.
 
(Ugh! I hate conics....)

You've got the equation:

. . . . .(x - a)<sup>2</sup> + (y - b)<sup>2</sup> = r<sup>2</sup>

They've given you (x, y) points, which leaves "a", "b", and "r" unknown. But they've given you three (x, y) points, which means you can plug in the x- and y-values, and get three equations. Then you can solve the system.

. . . . .a<sup>2</sup> + (-9 - b)<sup>2</sup> = r<sup>2</sup>

. . . . .(7 - a)<sup>2</sup> + (-2 - b)<sup>2</sup> = r<sup>2</sup>

. . . . .(-5 - a)<sup>2</sup> + (-10 - b)<sup>2</sup> = r<sup>2</sup>

The solving process will probably be messy, but it should be solvable. For instance, you would set the first two equations equal to each other, and solve for b in terms of a. Then work from there.

Eliz.
 
Top