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Geometry Difficulty 8.0 National olympiad, round 2 Find the answer

Let the intersections of O1\odot O_1 and O2\odot O_2 be AA and BB. Point RR is on arc ABAB of O1\odot O_1 and TT is on arc ABAB on O2\odot O_2. ARAR and BRBR meet O2\odot O_2 at CC and DD; ATAT and BTBT meet O1\odot O_1 at QQ and PP. If PRPR and TDTD meet at EE and QRQR and TCTC meet at FF, then prove: AEBTBR=BFATARAE \cdot BT \cdot BR = BF \cdot AT \cdot AR.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Let the intersections of O1\odot O_1 and O2\odot O_2 be AA and BB. Point RR is on arc ABAB of O1\odot O_1 and TT is on arc ABAB on O2\odot O_2. ARAR and BRBR meet O2\odot O_2 at CC and DD; ATAT and BTBT meet O1\odot O_1 at QQ and PP. If PRPR and TDTD meet at EE and QRQR and TCTC meet at FF, then we need to prove that AEBTBR=BFATARAE \cdot BT \cdot BR = BF \cdot AT \cdot AR.

First, note that from angle chasing, we have:
APE=ABR=ACD=ATD, \angle APE = \angle ABR = \angle ACD = \angle ATD,
which implies that AETPAETP is a cyclic quadrilateral. Similarly, we can show that AERDAERD, BFRCBFRC, and BFTQBFTQ are all cyclic quadrilaterals.

Next, observe that:
QRB=FCB=TDB, \angle QRB = \angle FCB = \angle TDB,
which implies that QRDTQR \parallel DT. Similarly, we have CTPRCT \parallel PR.

Additionally, note that:
TEA=πAPT=πTQB=BFT, \angle TEA = \pi - \angle APT = \pi - \angle TQB = \angle BFT,
and:
ATE=TQF=TBF, \angle ATE = \angle TQF = \angle TBF,
leading to AETBFT\triangle AET \sim \triangle BFT. This similarity implies:
ATBT=AETF. \frac{AT}{BT} = \frac{AE}{TF}.

By similar means, we have:
ARBR=AEFR. \frac{AR}{BR} = \frac{AE}{FR}.

Therefore, we obtain:
ATBTARBR=AETFAEFR=AEBF. \frac{AT}{BT} \cdot \frac{AR}{BR} = \frac{AE}{TF} \cdot \frac{AE}{FR} = \frac{AE}{BF}.

Since ETFRETFR is a parallelogram, we have ET=FRET = FR. Thus, we conclude that:
AEBTBR=BFATAR. AE \cdot BT \cdot BR = BF \cdot AT \cdot AR.

The answer is: AEBTBR=BFATAR\boxed{AE \cdot BT \cdot BR = BF \cdot AT \cdot AR}.

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