China Northern 2005-09, 2016-20 (CNMO) 20p (-08)

geometry problems from China Northern Mathematical Olympiads
with aops links in the names

China Sijiazhuang 

2005-07, 2009, 2016-20

missing 2008, 2010-2015

China Northern 2005 p1

$AB$ is a chord of a circle with center $O$, $M$ is the midpoint of $AB$. A non-diameter chord is drawn through $M$ and intersects the circle at $C$ and $D$. The tangents of the circle from points $C$ and $D$ intersect line $AB$ at $P$ and $Q$, respectively. Prove that $PA$ = $QB$.

China Northern 2006 p1

$AB$ is the diameter of circle $O$, $CD$ is a non-diameter chord that is perpendicular to $AB$. Let $E$ be the midpoint of $OC$, connect $AE$ and extend it to meet the circle at point $P$. Let $DP$ and $BC$ meet at $F$. Prove that $F$ is the midpoint of $BC$.

China Northern 2006 p3 (China TST 1989 p6)

$AD$ is the altitude of triangle $ABC$ at side $BC$. If $BC+AD=AB+AC$, then find the range of $\angle{A}$.

China Northern 2007 p1

Let $ABC$ be acute triangle. The circle with diameter $AB$ intersects $CA,\, CB$ at $M,\, N,$ respectively. Draw $CT\perp AB$ and intersects above circle at $T$, where $C$ and $T$ lie on the same side of $AB$. $S$ is a point on $AN$ such that $BT = BS$. Prove that $BS\perp SC$.

China Northern 2007 p8

The inradius of triangle $ABC$ is $1$ and the side lengths of $ABC$ are all integers. Prove that triangle $ABC$ is right-angled.

China Northern 2008 missing 

China Northern 2009 p2 

In an acute triangle $ABC$ , $AB>AC$ , $\cos B+ \cos C=1$ , $E,F$ are on the extend line of $AB,AC$ such that $\angle ABF = \angle ACE = 90$   .

(1) Prove :$BE+CF=EF$ ; 

(2) Assume the bisector of $\angle EBC$ meet $EF$ at $P$ , prove that $CP$ is the bisector of $\angle BCF$.

China Northern 2009 p6 

Given a minor sector $AOB$ (Here minor means that $\angle AOB <90$).  $O$ is the centre , chose a point  $C$  on arc $AB$ ,Let $P$ be a point on segment $OC$ , join $AP$ , $BP$ , draw a line through $B$ parallel to $AP$ , the line meet $OC$ at point $Q$ ,join $AQ$ . Prove that the area of polygon $AQPBO$ dose not change when points $P,C$ move .

China Northern 2010 missing 

China Northern  2011 p2

The incircle of $ABC$ is $(I)$,  $(I)$ touches $BC,CA,AB$ at $D,E,F$,  $P$ is a point inside the $(I)$,  $PA,PB,PC$ intersect $(I)$ at $X,Y,Z$. Prove $XD,YE,ZF$ are concurrent.

China Northern 2011 missing 

China Northern 2012 missing 

China Northern 2012 missing 

China Northern 2013 missing 

China Northern 2013 missing 

China Northern 2014 missing 

China Northern 2014 missing 

China Northern 2015 missing 

China Northern 2015 missing 

China Northern 2016 grade 10 p2
In isosceles triangle $ABC$, $\angle CAB=\angle CBA=\alpha$, points $P,Q$ are on different sides of line $AB$, and $\angle CAP=\angle ABQ=\beta,\angle CBP=\angle BAQ=\gamma$. Prove that $P,C,Q$ are collinear.

China Northern 2016 grade 10 p6 , 11 p6
Four points $B,E,A,F$ lie on line $AB$ in order, four points $C,G,D,H$ lie on line $CD$ in order, satisfying: $\frac{AE}{EB}=\frac{AF}{FB}=\frac{DG}{GC}=\frac{DH}{HC}=\frac{AD}{BC}.$ Prove that $FH\perp EG$.

China Northern 2016 grade 11 p2
Inscribed Triangle $ABC$ on circle $\odot O$. Bisector of $\angle ABC$ intersects $\odot O$ at $D$. Two lines $PB$ and $PC$ that are tangent to $\odot O$ intersect at $P$. $PD$ intersects $AC$ at $E$, $\odot O$ at $F$. Prove that $M,F,C,E$ are concyclic.

China Northern 2017 grade 10 p3 , 11 p2

Let $D$ be the midpoint of side $BC$ of triangle $ABC$. Let $E, F$ be points on sides $AB, AC$ respectively such that $DE = DF$. Prove that $AE + AF = BE + CF \iff \angle EDF = \angle BAC$.

China Northern 2017 grade 10 p5

Triangle $ABC$ has $AB > AC$ and $\angle A = 60^\circ$. Let $M$ be the midpoint of $BC$, $N$ be the point on segment $AB$ such that $\angle BNM = 30^\circ$. Let $D,E$ be points on $AB, AC$ respectively. Let $F, G, H$ be the midpoints of $BE, CD, DE$ respectively. Let $O$ be the circumcenter of triangle $FGH$. Prove that $O$ lies on line $MN$.

China Northern 2017 grade 11 p5

Length of sides of regular hexagon $ABCDEF$ is $a$. Two moving points $M,N$ moves on sides $BC,DE$, satisfy that $\angle MAN=\frac{\pi}{3}$. Prove that $AM\cdot AN-BM\cdot DN$ is a definite value.

China Northern 2018 grade 10 p1
In triangle $ABC$, let the circumcenter, incenter, and orthocenter be $O$, $I$, and $H$ respectively. Segments $AO$, $AI$, and $AH$ intersect the circumcircle of triangle $ABC$ at $D$, $E$, and $F$. $CD$ intersects $AE$ at $M$ and $CE$ intersects $AF$ at $N$. Prove that $MN$ is parallel to $BC$.

China Northern 2018 grade 10 p6 , 11 p5
Let $H$ be the orthocenter of triangle $ABC$. Let $D$ and $E$ be points on $AB$ and $AC$ such that $DE$ is parallel to $CH$. If the circumcircle of triangle $BDH$ passes through $M$, the midpoint of $DE$, then prove that $\angle ABM=\angle ACM$


China Northern 2018 grade 11 p3
$A,B,C,D,E$ lie on $\odot O$ in that order,and $$BD \cap CE=F,CE \cap AD=G,AD \cap BE=H,BE \cap AC=I,AC \cap BD=J.$$ Prove that $\frac{FG}{CE}=\frac{GH}{DA}=\frac{HI}{BE}=\frac{IJ}{AC}=\frac{JF}{BD}$ when and only when $F,G,H,I,J$ are concyclic.

China Northern 2019 p2

Two circles $O_1$ and $O_2$ intersect at $A,B$. Diameter $AC$ of $\odot O_1$ intersects $\odot O_2$ at $E$, Diameter $AD$ of $\odot O_2$ intersects $\odot O_1$ at $F$. $CF$ intersects $O_2$ at $H$, $DE$ intersects $O_1$ at $G,H$. $GH\cap O_1=P$. Prove that $PH=PK$.

China Northern 2019 p5

Two circles $O_1$ and $O_2$ intersect at $A,B$. Bisector of outer angle $\angle O_1AO_2$ intersects $O_1$ at $C$, $O_2$ at $D$. $P$ is a point on $\odot(BCD)$, $CP\cap O_1=E,DP\cap O_2=F$. Prove that $PE=PF$.

China Northern 2020 basic p2

In $\triangle ABC$, $\angle BAC = 60^{\circ}$, point $D$ lies on side $BC$, $O_1$ and $O_2$ are the centers of the circumcircles of $\triangle ABD$ and $\triangle ACD$, respectively. Lines $BO_1$ and $CO_2$ intersect at point $P$. If $I$ is the incenter of $\triangle ABC$ and $H$ is the orthocenter of $\triangle PBC$, then prove that the four points $B,C,I,H$ are on the same circle.

China Northern 2020 advanced p2

In $\triangle ABC$, $AB>AC$. Let $O$ and $I$ be the circumcenter and incenter respectively. Prove that if $\angle AIO = 30^{\circ}$, then $\angle ABC = 60^{\circ}$.

before 2016 mentioned in aops