Theorem 3.12: The point where the line joining the centers of two circles meets the line containing the points of intersection of the two circles is given by
and
where (x1, y1) and (x2, y2) are the centers of the circles and r1 and r2 are the respective radii.
Proof: Case 1: x1 = x2. In this case the equation of the line joining the centers is x = x1. From Theorem 3.10, the equation containing the points of intersection will be
after we simplify the formula by taking into account that x1 = x2. In this case the point of intersection will be
Case 2: y1 = y2. In this case the equation of the line joining the centers is y = y1. From Theorem 3.10, the equation containing the points of intersection will be
so the point of intersection will be
If y1 = y2, then the formulas in the statement of the theorem simplify to give these coordinates.
Case 3: The line joining the centers is neither horizontal nor vertical. Since the line containing the solutions is perpendicular to the line joining the centers, by Theorem 3.11, we know the point of intersection will be the foot of (x1, y1) in the line containing the solutions, which by Theorem 3.10 is
We can then use Theorem 1.10 to get the coordinates of the foot as
where
and
Substituting this in to the formula for the x-coordinate of the foot gives us
First let find common denominators for the first two terms on the top and clean up the last term.
The first fraction on the top will simplify. Remove parentheses on the top.
As we cancel the like terms, let us rearrange the terms in the second factor of the top of the second fraction on the top.
To clear denominators in the compound fraction, we multiply the top and bottom by 2(y2 - y1)2.
We will split the second term up as
Factor in the middle two terms.
We can now factor out y2 - y1 in the first two terms in the top and clean up the third term.
Remove parentheses in the top of the first fraction.
Combine like terms
The top of the first fraction factors
Factor out the common factors from the first two fractions.
This simplifies to
We could use a similar approach to find the y-coordinate, but it might be simpler to use the fact that the point is on the line joining the centers, and using the equation for the line that joins (x1, y1) and (x2, y2), which, since we have voided the case where the centers of the circle are vertical, is of the form y = mx + b by Theorem 1.2.
This becomes
or
In the first two terms, we are substituting the x-coordinate for the midpoint of the line segment between (x1, y1) and (x2, y2) into the equation for the line that they determine, and so we should get the y coordinate of that same midpoint which by Theorem 2.4 is
By Theorem 1.4
This gives us
or
