2005 AIME II Problems/Problem 15
Problem
Let and
denote the circles
and
respectively. Let
be the smallest positive value of
for which the line
contains the center of a circle that is externally tangent to
and internally tangent to
Given that
where
and
are relatively prime integers, find
Solution
Rewrite the given equations as and
.
Let have center
and radius
. Now, if two circles with radii
and
are externally tangent, then the distance between their centers is
, and if they are internally tangent, it is
. So we have
Solving for in both equations and setting them equal, then simplifying, yields
Squaring again and canceling yields
So the locus of points that can be the center of the circle with the desired properties is an ellipse.
![[asy] size(220); pointpen = black; pen d = linewidth(0.7); pathpen = d; pair A = (-5, 12), B = (5, 12), C = (0, 0); D(CR(A,16));D(CR(B,4));D(shift((0,12)) * yscale(3^.5 / 2) * CR(C,10), linetype("2 2") + d + red); D((0,30)--(0,-10),Arrows(4));D((15,0)--(-25,0),Arrows(4));D((0,0)--MP("y=ax",(14,14 * (69/100)^.5),E),EndArrow(4)); void bluecirc (real x) { pair P = (x, (3 * (25 - x^2 / 4))^.5 + 12); dot(P, blue); D(CR(P, ((P.x - 5)^2 + (P.y - 12)^2)^.5 - 4) , blue + d + linetype("4 4")); } bluecirc(-9.2); bluecirc(-4); bluecirc(3); [/asy]](http://latex.artofproblemsolving.com/3/9/5/3952f7611dcec95c9d68289eb69eb74de86b62bc.png)
Since the center lies on the line , we substitute for
and expand:
We want the value of that makes the line
tangent to the ellipse, which will mean that for that choice of
there is only one solution to the most recent equation. But a quadratic has one solution iff its discriminant is
, so
.
Solving yields , so the answer is
.
See also
2005 AIME II (Problems • Answer Key • Resources) | ||
Preceded by Problem 14 |
Followed by Last Question | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
All AIME Problems and Solutions |