# 2001 AMC 12 Problems/Problem 18

## Problem

A circle centered at $A$ with a radius of 1 and a circle centered at $B$ with a radius of 4 are externally tangent. A third circle is tangent to the first two and to one of their common external tangents as shown. The radius of the third circle is

$[asy] unitsize(0.75cm); pair A=(0,1), B=(4,4); dot(A); dot(B); draw( circle(A,1) ); draw( circle(B,4) ); draw( (-1.5,0)--(8.5,0) ); draw( A -- (A+(-1,0)) ); label("1", A -- (A+(-1,0)), N ); draw( B -- (B+(4,0)) ); label("4", B -- (B+(4,0)), N ); label("A",A,E); label("B",B,W); filldraw( circle( (12/9,4/9), 4/9 ), lightgray, black ); dot( (12/9,4/9) ); [/asy]$

$\text{(A) }\frac {1}{3} \qquad \text{(B) }\frac {2}{5} \qquad \text{(C) }\frac {5}{12} \qquad \text{(D) }\frac {4}{9} \qquad \text{(E) }\frac {1}{2}$

## Solution

### Solution 1

$[asy] unitsize(1cm); pair A=(0,1), B=(4,4), C=(4,1); dot(A); dot(B); draw( circle(A,1) ); draw( circle(B,4) ); draw( (-1.5,0)--(8.5,0) ); draw( (A+(4,0)) -- A -- (A+(0,-1)) ); draw( A -- B -- (B+(0,-4)) ); label("A",A,N); label("B",B,N); label("C",C,E); filldraw( circle( (12/9,4/9), 4/9 ), lightgray, black ); dot( (12/9,4/9) ); draw( rightanglemark(A,C,B) ); [/asy]$

In the triangle $ABC$ we have $AB = 1+4 = 5$ and $BC=4-1 = 3$, thus by the Pythagorean theorem we have $AC=4$.

We can now pick a coordinate system where the common tangent is the $x$ axis and $A$ lies on the $y$ axis. In this coordinate system we have $A=(0,1)$ and $B=(4,4)$.

Let $r$ be the radius of the small circle, and let $s$ be the $x$-coordinate of its center $S$. We then know that $S=(s,r)$, as the circle is tangent to the $x$ axis. Moreover, the small circle is tangent to both other circles, hence we have $SA=1+r$ and $SB=4+r$.

We have $SA = \sqrt{s^2 + (1-r)^2}$ and $SB=\sqrt{(4-s)^2 + (4-r)^2}$. Hence we get the following two equations:

\begin{align*} s^2 + (1-r)^2 & = (1+r)^2 \\ (4-s)^2 + (4-r)^2 & = (4+r)^2 \end{align*}

Simplifying both, we get

\begin{align*} s^2 & = 4r \\ (4-s)^2 & = 16r \end{align*}

As in our case both $r$ and $s$ are positive, we can divide the second one by the first one to get $\left( \frac{4-s}s \right)^2 = 4$.

Now there are two possibilities: either $\frac{4-s}s=-2$, or $\frac{4-s}s=2$. In the first case clearly $s<0$, hence this is not the correct case. (Note: This case corresponds to the other circle that is tangent to both given circles and the $x$ axis - a large circle whose center is somewhere to the left of $A$.) The second case solves to $s=\frac 43$. We then have $4r = s^2 = \frac {16}9$, hence $r = \boxed{\frac 49}$.

### Solution 2

The horizontal line is the equivalent of a circle of curvature $0$, thus we can apply Descartes' Circle Formula.

The four circles have curvatures $0, 1, \frac 14$, and $\frac 1r$.

We have $2(0^2+1^2+\frac {1}{4^2}+\frac{1}{r^2})=(0+1+\frac 14+\frac 1r)^2$

Simplifying, we get $\frac{34}{16}+\frac{2}{r^2}=\frac{25}{16}+\frac{5}{2r}+\frac{1}{r^2}$

$\frac{1}{r^2}-\frac{5}{2r}+\frac{9}{16}=0$

$\frac{16}{r^2}-\frac{40}{r}+9=0$

$(\frac{4}{r}-9)(\frac{4}{r}-1)=0$

Obviously $r$ cannot equal $4$, therefore $r = \boxed{\frac 49}$.