In triangle ABC, arbitrary points P,Q lie on side BC such that BP=CQ and P lies between B,Q. The circumcircle of triangle APQ intersects sides AB and AC at E and F, respectively. The point T is the intersection point of EP and FQ. Two lines passing through the midpoint of BC and parallel to AB and AC, intersect EP and FQ at points X and Y, respectively. Prove that the circumcircles of triangles TXY and APQ are tangent to each other.
Solution
Let M be the midpoint of BC. Since BP=CQ, it is clear that MP=MQ and BQ=CP. Let Z be the second intersection point of circumcircles of triangles MPX and MQY. We claim that the circumcircles of triangles APQ and XYT are tangent to each other at Z.
We have ⎩⎨⎧PZQ=PZM+QZM,C△MQY:QZM=MYQ=QFC,C△APQ:QFC=APQ,C△MPX:PZM=MXP=PEB,C△APQ:PEB=AQP,
And so PZQ=APQ+AQP=180∘−PAQ. Which means Z lies on the circumcircle of triangle APQ. Also {C△MQY:MZY=180∘−MQT,C△MPX:MZX=180∘−MPT. These imply XYZ=360∘−(MZY+MZX)=MQT+MPT=180∘−PTQ. Which means Z also lies on the circumcircle of triangle XYZ. We use a lemma to prove the problem.
Lemma. Given two circles Ω and Γ with a common point X and a line intersecting Ω at Y and P, and intersecting Γ at Q and Z (P and Q lie between Y and Z). In this case, these circles are tangent to each other at X if PXQ=XYZ+XZY.
Proof. Let l be the tangent line to Ω through X, therefore PXl=XYP (with a fixed direction on l, and angles are considered with respect to this direction). l is also tangent to Γ, if and only if XZQ=QXl. But PXl=PXQ−QXl. Therefore XZQ=PXQ−XYZ=PXQ−PXl=QXl.
Now back to the problem, according to the lemma (for two circles C△MQY, C△APQ), it suffices to show that QZY=ZFT+ZTF. We have ⎩⎨⎧QZY=QMY=C^,C△TXY:ZTF=ZXY,C△APQ:TFZ=QPZ,C△MPX:QPZ=MXZ. Note that XMY=A^ and the problem is equivalent to prove that C=MXZ+ZXY=MXY⟺MXY⟺MXMY∼ACB,=ACAB. We also have: MY∥AC⟹FCMY=QCMQMX∥AB⟹EBMX=PBMPMP=MQ,BP=CQ⎭⎬⎫⟹MXMY=EBFC. So the problem is equivalent to show that EBFC=ACAB, or equivalently FC⋅AC=EB⋅AB. In order to prove this, we have CF⋅CA=PC△APQ(C)=CQ⋅CP=BP⋅BQ=PC△APQ(B)=BE⋅BA, (Where Pλ(S) is the power of point S with respect to circle λ.) Hence the claim. ■
Want a route through all this instead of an archive? The track
puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.
Source: MathNet,
licensed CC-BY-4.0.
Statement reproduced verbatim; metadata (topic, difficulty) added by this project.