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Geometry Difficulty 7.3 National Olympiad, round 2 Prove it Iran

In triangle ABCABC, arbitrary points P,QP, Q lie on side BCBC such that BP=CQBP = CQ and PP lies between B,QB, Q. The circumcircle of triangle APQAPQ intersects sides ABAB and ACAC at EE and FF, respectively. The point TT is the intersection point of EPEP and FQFQ. Two lines passing through the midpoint of BCBC and parallel to ABAB and ACAC, intersect EPEP and FQFQ at points XX and YY, respectively. Prove that the circumcircles of triangles TXYTXY and APQAPQ are tangent to each other.

Solution

Let MM be the midpoint of BCBC. Since BP=CQBP = CQ, it is clear that MP=MQMP = MQ and BQ=CPBQ = CP. Let ZZ be the second intersection point of circumcircles of triangles MPXMPX and MQYMQY. We claim that the circumcircles of triangles APQAPQ and XYTXYT are tangent to each other at ZZ.

Figure 1

We have
{PZQ^=PZM^+QZM^,CMQY:QZM^=MYQ^=QFC^,CAPQ:QFC^=APQ^,CMPX:PZM^=MXP^=PEB^,CAPQ:PEB^=AQP^, \left\{ \begin{array}{l} \widehat{PZQ} = \widehat{PZM} + \widehat{QZM}, \\ C_{\triangle MQY} : \widehat{QZM} = \widehat{MYQ} = \widehat{QFC}, \\ C_{\triangle APQ} : \widehat{QFC} = \widehat{APQ}, \\ C_{\triangle MPX} : \widehat{PZM} = \widehat{MXP} = \widehat{PEB}, \\ C_{\triangle APQ} : \widehat{PEB} = \widehat{AQP}, \end{array} \right.

And so PZQ^=APQ^+AQP^=180PAQ^\widehat{PZQ} = \widehat{APQ} + \widehat{AQP} = 180^\circ - \widehat{PAQ}. Which means ZZ lies on the circumcircle of triangle APQAPQ. Also
{CMQY:MZY^=180MQT^,CMPX:MZX^=180MPT^. \begin{cases} C_{\triangle MQY} : \widehat{MZY} = 180^\circ - \widehat{MQT}, \\ C_{\triangle MPX} : \widehat{MZX} = 180^\circ - \widehat{MPT}. \end{cases}
These imply
XYZ^=360(MZY^+MZX^)=MQT^+MPT^=180PTQ^. \widehat{XYZ} = 360^\circ - (\widehat{MZY} + \widehat{MZX}) = \widehat{MQT} + \widehat{MPT} = 180^\circ - \widehat{PTQ}.
Which means ZZ also lies on the circumcircle of triangle XYZXYZ. We use a lemma to prove the problem.

Lemma. Given two circles Ω\Omega and Γ\Gamma with a common point XX and a line intersecting Ω\Omega at YY and PP, and intersecting Γ\Gamma at QQ and ZZ (PP and QQ lie between YY and ZZ). In this case, these circles are tangent to each other at XX if
PXQ^=XYZ^+XZY^. \widehat{PXQ} = \widehat{XYZ} + \widehat{XZY}.
Figure 2

Proof. Let ll be the tangent line to Ω\Omega through XX, therefore PXl^=XYP^\widehat{PXl} = \widehat{XYP} (with a fixed direction on ll, and angles are considered with respect to this direction). ll is also tangent to Γ\Gamma, if and only if XZQ^=QXl^\widehat{XZQ} = \widehat{QXl}. But PXl^=PXQ^QXl^\widehat{PXl} = \widehat{PXQ} - \widehat{QXl}. Therefore
XZQ^=PXQ^XYZ^=PXQ^PXl^=QXl^. \widehat{XZQ} = \widehat{PXQ} - \widehat{XYZ} = \widehat{PXQ} - \widehat{PXl} = \widehat{QXl}.

Now back to the problem, according to the lemma (for two circles CMQYC_{\triangle MQY}, CAPQC_{\triangle APQ}), it suffices to show that
QZY^=ZFT^+ZTF^. \widehat{QZY} = \widehat{ZFT} + \widehat{ZTF}.
We have
{QZY^=QMY^=C^,CTXY:ZTF^=ZXY^,CAPQ:TFZ^=QPZ^,CMPX:QPZ^=MXZ^. \left\{ \begin{array}{l} \widehat{QZY} = \widehat{QMY} = \hat{C}, \\ C_{\triangle TXY} : \widehat{ZTF} = \widehat{ZXY}, \\ C_{\triangle APQ} : \widehat{TFZ} = \widehat{QPZ}, \\ C_{\triangle MPX} : \widehat{QPZ} = \widehat{MXZ}. \end{array} \right.
Note that XMY^=A^\widehat{XMY} = \hat{A} and the problem is equivalent to prove that
C^=MXZ^+ZXY^=MXY^    MXYACB^,    MYMX=ABAC. \begin{align*} \widehat{C} &= \widehat{MXZ} + \widehat{ZXY} = \widehat{MXY} &\iff MXY &\sim \widehat{ACB}, \\ & &\iff \frac{MY}{MX} &= \frac{AB}{AC}. \end{align*}
We also have:
MYAC    MYFC=MQQCMXAB    MXEB=MPPBMP=MQ,BP=CQ}    MYMX=FCEB. \left. \begin{array}{l} MY \parallel AC \implies \frac{MY}{FC} = \frac{MQ}{QC} \\ MX \parallel AB \implies \frac{MX}{EB} = \frac{MP}{PB} \\ MP = MQ, \quad BP = CQ \end{array} \right\} \implies \frac{MY}{MX} = \frac{FC}{EB}.
So the problem is equivalent to show that FCEB=ABAC\frac{FC}{EB} = \frac{AB}{AC}, or equivalently FCAC=EBABFC \cdot AC = EB \cdot AB. In order to prove this, we have
CFCA=PCAPQ(C)=CQCP=BPBQ=PCAPQ(B)=BEBA, CF \cdot CA = \mathcal{P}_{C_{\triangle APQ}}(C) = CQ \cdot CP = BP \cdot BQ = \mathcal{P}_{C_{\triangle APQ}}(B) = BE \cdot BA,
(Where Pλ(S)\mathcal{P}_\lambda(S) is the power of point SS with respect to circle λ\lambda.) Hence the claim. ■

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