Let $ABCD$ a quadrilateral inscribed in a circumference, $l_1$ the parallel to $BC$ through $A$, and $l_2$ the parallel to $AD$ through $B$. The line $DC$ intersects $l_1$ and $l_2$ at $E$ and $F$, respectively. The perpendicular to $l_1$ through $A$ intersects $BC$ at $P$, and the perpendicular to $l_2$ through $B$ cuts $AD$ at $Q$. Let $\Gamma_1$ and $\Gamma_2$ be the circumferences that pass through the vertex of triangles $ADE$ and $BFC$, respectively. Prove that $\Gamma_1$ and $\Gamma_2$ are tangent to each other if and only if $DP$ is perpendicular to $CQ$.
Problem
Source: Mexican National Olympiad 2016
Tags: Mexico, geometry, tangent circles
juckter
20.11.2016 04:56
Notice that $\measuredangle EDA = \measuredangle CDA = \measuredangle CBA = \measuredangle EAB$ (directed angles), and hence $AB$ is tangent to $\Gamma_1$. Similarly, $AB$ is tangent to $\Gamma_2$.
Now notice that since $AP \perp BC$ and $AD \perp BQ$, the contition $PD \perp CQ$ happens if and only if triangles $APD$ and $BCQ$ are similar. This in turn happens if and only if $\frac{AP}{AD} = \frac{BC}{CQ}$, if and only if $AP \cdot CQ = BC \cdot AD$. Now notice that triangles $ADE$ and $BFC$ are similar, and hence $\frac{AD}{AE} = \frac{BF}{BC} \implies AD \cdot BC = AE \cdot BF$. Thus the previous condition turns into $AP \cdot CQ = AE \cdot BF$ which happens if and only if $\frac{AP}{AE} = \frac{BF}{BQ}$. And since we also have $\angle PAE = \angle FBQ = 90^{\circ}$, this in turn happens if and only if $\triangle PAE \sim \triangle FBQ$.
Now assume that $\Gamma_1$ and $\Gamma_2$ are tangent at $R$. Straightforward angle manipulation involving the centers of $\Gamma_1$ and $\Gamma_2$ shows that $\angle ARB = 90^{\circ}$ follows from $AB$ being an external common tangent to $\Gamma_1$ and $\Gamma_2$. Thus $A, Q, P, B, R$ are all concyclic on the circle with diameter $AB$. Thus
$$\measuredangle ARP = \measuredangle ABP = \measuredangle ABC = \measuredangle ADC = \measuredangle ADE= \measuredangle ARE$$And thus $E, R, P$ are collinear. Analogously $F, R, Q$ are collinear, hence we have
$$\measuredangle PEA = \measuredangle REA = \measuredangle RAB = \measuredangle RQB = \measuredangle FQB$$Thus triangles $PEA$ and $FQB$ and similar.
Now assume triangles $PEA$ and $FQB$ are similar and let $S$ be the intersection point of $PE$ and $QF$, we shall prove that $\Gamma_1$ and $\Gamma_2$ are tangent at $S$. Due to the similarity we have
$$\measuredangle SQB = \measuredangle FQB = \measuredangle PEA = \measuredangle EPC = \measuredangle SPB$$
Hence $S, Q, B, P$ are concyclic and thus $A, Q, P, B, S$ are all concyclic. Now we have
$$\measuredangle ASE = \measuredangle ASP = \measuredangle ABP = \measuredangle ABC = \measuredangle ADC = \measuredangle ADE$$
And thus $S$ lies on $\Gamma_1$. Analogously $S$ lies on $\Gamma_2$. Finally:
$$\measuredangle PSA = 90^{\circ} = \measuredangle SAB + \measuredangle ABS = \measuredangle SDA +\measuredangle BCS$$
So the tangent line to $\Gamma_1$ at $S$ is also tangent to $\Gamma_2$, and hence $\Gamma_1$ and $\Gamma_2$ are tangent at $S$.