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Let $ABC$ be a triangle. Its incircle $\omega$ touches the sides $BC, CA$ and $AB$ at points $D, E$ and $F$, respectively. Let $P$ and $Q$ be points on the line $BC$ distinct from $D$ such that $PB = BD$ and $QC = CD$. Prove that the circumcircles of the triangles $PCE$ and $QBF$ and the circle $\omega$ pass through a common point.

Let $ABC$ be an acute triangle with $AB < AC$. Denote by $D$ the foot of the perpendicular from $A$ to $BC$. Let $E$ be the point such that $ABEC$ is a parallelogram. Let $M$ be a point inside triangle $ABC$ such that $MB = MC$. Let $F$ be the reflection of point $D$ across the tangent to the circumcircle of triangle $ADM$ at point $M$. Prove that $AF = DE$.

Let $ABC$ be an acute triangle with an interior point $D$ such that $\angle BDC = 180^{\circ} - \angle BAC$. The lines $BD$ and $AC$ intersect at the point $E$, and the lines $CD$ and $AB$ intersect at the point $F$. The points $P \neq E$ and $Q \neq F$ lie on the line $EF$ so that $BP = BE$ and $CQ = CF$. Assume that the segments $AP$ and $AQ$ intersect the circumcircle $\omega$ of $ABC$ at the points $R \neq A$ and $S \neq A$, respectively. Prove that the lines $RF$ and $SE$ intersect on $\omega$.

Neka je $M$ točka unutar trokuta $ABC$ na simetrali kuta $\measuredangle BAC$. Pravci $AM$, $BM$ i $CM$ ponovo sijeku opisanu kružnicu trokuta $ABC$ redom u točkama $A_1$, $B_1$ i $C_1$. Neka je $P$ sjecište dužina $\overline{A_1C_1}$ i $\overline{AB}$ te $Q$ sjecište dužina $\overline{A_1B_1}$ i $\overline{AC}$.

Dokaži da su pravci $PQ$ i $BC$ paralelni.

Neka je $H$ ortocentar šiljastokutnog trokuta $ABC$ i $M$ polovište stranice $\overline{AB}$. Pravac $HM$ siječe pravce $AC$ i $BC$ redom u točkama $A_1$ i $B_1$. Neku su $A_2$ i $B_2$ redom nožišta okomica iz $A_1$ i $B_1$ na pravac $CH$. Dokaži da se pravci $AB_2$ i $BA_2$ sijeku na opisanoj kružnici trokuta $ABC$.

Neka je $D$ točka unutar trokuta $ABC$ i neka je $E$ točka na dužini $\overline{AD}$ različita od $A$ i $D$. Opisane kružnice trokuta $BDE$ i $CDE$ sijeku stranicu $\overline{BC}$ redom u točkama $F$ i $G$. Neka je $X$ sjecište pravaca $DG$ i $AB$, a $Y$ sjecište pravaca $DF$ i $AC$.

Dokaži da su pravci $XY$ i $BC$ paralelni.

Neka je $ABC$ šiljastokutan trokut u kojemu je $|AB| > |AC|$, točka $I$ središte njemu upisane kružnice, a $P$ polovište dužine $\overline{BC}$. Neka je $K$ polovište luka $\widehat{BC}$ kružnice opisane trokutu $ABC$ koji sadrži točku $A$. Dokaži da vrijedi $\measuredangle BIP + \measuredangle CIK = 180°$.

Neka je $ABC$ trokut s pravim kutom u vrhu $C$. Neka je $D$ nožište visine iz vrha $C$. Kružnica sa središtem u $C$ polumjera $|CD|$ siječe opisanu kružnicu trokuta $ABC$ u točkama $E$ i $F$. Pravac $EF$ siječe dužinu $\overline{CD}$ u točki $P$. Dokaži da je $P$ polovište dužine $\overline{CD}$.