Problem 1685

1. Define the setup and key points:
- Let $\omega_1, \omega_2, \omega_3$ be the three circles tangent to line $l$ at points $A, B, C$ respectively, with $B$ lying between $A$ and $C$.
- $\omega_2$ is externally tangent to both $\omega_1$ and $\omega_3$.
- Let $X$ and $Y$ be the intersection points of $\omega_2$ with the other common external tangent of $\omega_1$ and $\omega_3$.
- The perpendicular line through $B$ to $l$ meets $\omega_2$ again at $Z$.

2. **Identify the intersection point $J$:**
- Let $J$ be the intersection point of $AC$ and $XY$.
- Let $O_1$ and $O_3$ be the centers of $\omega_1$ and $\omega_3$ respectively.
- Since $J, O_1, O_3$ are collinear, we can use this collinearity in our proof.

3. **Consider the inversion centered at $J$:**
- This inversion sends $\omega_1$ to $\omega_3$ and vice versa, and it also sends $AC$ to itself.
- Since $\omega_2$ is tangent to both $\omega_1$ and $\omega_3$ and $AC$, it must be fixed under this inversion.
- Therefore, $B$ is sent to $B$, and $D$ (the point of tangency between $\omega_1$ and $\omega_2$) is sent to $G$ (the point of tangency between $\omega_2$ and $\omega_3$).

4. Prove collinearity and power of point:
- Since $J, D, G$ are collinear and $JB^2 = JA \cdot JC$, we can use this relationship in our proof.

5. Analyze the tangents and circles:
- Let the common tangent of $\omega_1$ and $\omega_2$ meet $AB$ at $N$. Then $NA = ND = NB$, so $D$ lies on the circle with diameter $AB$.
- Since $\angle ADB = 90^\circ$, we have $\angle ADB + \angle BDZ = 180^\circ$, implying that $A, D, Z$ are collinear. Similarly, $C, G, Z$ are collinear.

6. **Consider the tangents from $Z$ to the circle with diameter $AC$:**
- Let $X'Z$ and $Y'Z$ be the tangents from $Z$ to the circle with diameter $AC$, touching it at $R$ and $P$ respectively.
- Let $K = RP \cap AC$. By La Hire's theorem, the polar of $K$ with respect to the circle with diameter $AC$ goes through $Z$.
- Therefore, $ZB$ is the polar of $K$ with respect to the circle with diameter $AC$, making $K$ the inverse of $B$ with respect to this circle.

7. Use harmonic division:
- It is well known that $(K, B; A, C) = -1$. Let $K'$ be the reflection of $B$ with respect to $J$.
- Since $JB^2 = JA \cdot JC$, inversion with respect to $(K'B)$ takes $A$ to $C$, implying $(K', B; A, C) = -1$, so $K = K'$.
- Thus, $J$ is the midpoint of $KB$.

8. Prove the lemma:
- Let $A, B, C, D$ be four points on a circle $\omega$. Tangents to $\omega$ at $A$ and $D$ meet at $M$, and tangents to $\omega$ at $B$ and $C$ meet at $N$.
- Let $J = AB \cap CD$. By Brocard's theorem, $JF$ is the polar of $E$, and by La Hire's theorem, the polar of $E$ goes through $M$ and $N$, making $J, M, F, N$ collinear.

9. Conclude the proof:
- Let $DG$ meet $(AB)$ and $(BC)$ again at $E$ and $F$ respectively. Let $ZB \cap O_1O_2 = O$.
- Since $O_1A, O_1D, OB$ are tangent to $(AB)$, by our lemma, $OE$ is tangent to $(AB)$. Similarly, $OF$ is tangent to $(BC)$, implying $OB = OE = OF$, making $O$ the circumcenter of $\triangle EBF$.
- Let $M$ be the midpoint of $AC$. Since $\{R, B, P\} \in (ZM)$, we have $\angle EAB = \angle EDB = \angle BZC$.
- Since $BZ \perp AC$, $AE \perp ZC$. Similarly, $CF \perp ZA$, so $CF$ and $AZ$ meet at $(AC)$.
- $AE, CF, ZB$ concur at the orthocenter $H$ of $\triangle ZAC$. The inversion with center $Z$ and radius $ZR$ takes $H$ to $B$.
- Since $ZRBMP$ is cyclic, $H \in RP$. Let $I \in RP$ such that $BI \perp RP$. By Simson's theorem, $I \in X'Y'$.
- Since $\angle X'IB = \angle X'RB = \angle ZMB = \angle IBK$, $X'IY'$ goes through the midpoint $J$ of $KB$.
- Since $\{E, I, F\} \in (HB)$, $OI = OB$. Since $JI = JB$, the line $JX'IY'$ is the reflection of $AC$ with respect to the line $O_1O_3$.
- Therefore, $X'Y'$ is the other common external tangent of $\omega_1$ and $\omega_3$.

$\blacksquare$