2015 AMC 12B Problems/Problem 24
Four circles, no two of which are congruent, have centers at , , , and , and points and lie on all four circles. The radius of circle is times the radius of circle , and the radius of circle is times the radius of circle . Furthermore, and . Let be the midpoint of . What is ?
First, note that lies on the radical axis of any of the pairs of circles. Suppose that and are the centers of two circles and that intersect exactly at and , with and lying on the same side of , and . Let , , and suppose that the radius of circle is and the radius of circle is .
Then the power of point with respect to is
and the power of point with respect to is
Also, note that .
Subtract the above two equations to find that or . As , we find that . Plug this into an earlier equation to find that . This is a quadratic equation with solutions , and as is a length, it is positive, hence , and . This is the only possibility if the two centers lie on the same side of their radical axis.
On the other hand, if they lie on opposite sides, then it is clear that there is only one possibility, and then it is clear that . Therefore, we obtain exactly four possible centers, and the sum of the desired lengths is .
Note that if a circle passes through a pair of points, the center of the circle is on the perpendicular bisector of the line segment between the pair of points. This means that , , , and are all on the perpendicular bisector of . Let us say the distance from to the line is some . Therefore, the distance from to the line is , which comes out to be . Since , we have one of to be equal to . We can solve both equations to get that out of the four possible solutions, and only two are positive: and . Note that since no two circles can be congruent, we need the radius of one of or to be and the other to be . Plugging in to find the corresponding radii of and gives and , and adding everything up gives . ~Solution by hyxue
Start by drawing first, because trying to get all four circles down will take you a few years. Next, because all circles have and on them, and since all points on a circle are equidistant from the center, all circle centers lie on the perpendicular bisector of , and point is on this bisector.
In order for all the circle radii to be different (because the circles can't be congruent), two circle centers are on the same side of , and two are straddling it. For the latter two circles- just call them and - clearly is 39.
Now, let's take the next case. Then and lie on the same side. Construct the triangles from your picture, and use the Pythagorean Theorem (maybe divide all lengths by 3 to lessen big numbers) and then you get that the distance from to the closest circle center is . Therefore, the answer is .
Pythagorean Theorem Solution
Since the radical axis is perpendicular to the line connecting the center of the circles, we have that and are collinear. WLOG, assume that and are on the same side of . Let and let so that . Then, we have in right triangles and , Subtracting the second from the first gives and substituting this in the second equation gives Since and while Since none of the circles are congruent, and must be on the opposite side of so Hence, which is .
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