Maths Olympiad Prep

Library / /13 of 18

Geometry Difficulty 7.4 National olympiad, round 2 Prove it China

Let ABC\triangle ABC be an acute triangle, and let KK be a point on the extension ray of BCBC (so that CC lies on the segment BKBK). Let PP be a point such that BP=BKBP = BK and PKABPK \parallel AB, and let QQ be a point such that CQ=CKCQ = CK and QKACQK \parallel AC. Assume that the circumcircle of PQK\triangle PQK and the line AKAK intersects at another point TT.

(1) Prove that APB+BTC=CQA\angle APB + \angle BTC = \angle CQA.

(2) Prove that APBTCQ=AQBPCTAP \cdot BT \cdot CQ = AQ \cdot BP \cdot CT.

Solution

Proof 1. (1) Let A1A_1 be the reflection of AA across BB. By the given conditions, KPA1AKPA_1A forms an isosceles trapezoid, thus APB=A1KB\angle APB = \angle A_1KB.
Similarly, let A2A_2 be the reflection of AA across CC. We have AQC=A2KC=A2KB\angle AQC = \angle A_2KC = \angle A_2KB.
Hence, the desired conclusion is equivalent to BTC=A1KA2\angle BTC = \angle A_1KA_2.
Perform a homothety centered at AA with a ratio of 1/21/2 on A1KA2\triangle A_1KA_2, mapping A1A_1 to BB, A2A_2 to CC, and KK to the midpoint of AKAK (denoted as MM). The proposition then transforms to BTC=BMC\angle BTC = \angle BMC, i.e., points B,C,MB, C, M, and TT are concyclic.
By the Power of a Point Theorem, this is equivalent to proving KBKC=KMKTKB \cdot KC = KM \cdot KT. If we take XX as the midpoint of KTKT, it is equivalent to proving KBKC=KAKXKB \cdot KC = KA \cdot KX, i.e., points A,B,CA, B, C, and XX are concyclic.
We note that the perpendicular bisector of segment KPKP passes through point BB and is perpendicular to ABAB, and the perpendicular bisector of KQKQ passes through CC and is perpendicular to ACAC. Hence, the circumcenter of PQK\triangle PQK (denoted as OO) is the antipodal point of AA on the unit circle. From OXKTOX \perp KT, we have OXA=π2\angle OXA = \frac{\pi}{2}, and thus XX lies on the circle with diameter AOAO (the circumcircle of ABC\triangle ABC). Proof completed.

Figure 1

(2) We have
BPCQ=BKCK=SBTKSCTK=BTCTsinBTKsinCTK=BTCTsinMCKsinMBK=BTCTBMCM=BTCTKA1KA2=BTCTAPAQ. \begin{aligned} \frac{BP}{CQ} &= \frac{BK}{CK} = \frac{S_{\triangle BTK}}{S_{\triangle CTK}} = \frac{BT}{CT} \cdot \frac{\sin \angle BTK}{\sin \angle CTK} = \frac{BT}{CT} \cdot \frac{\sin \angle MCK}{\sin \angle MBK} \\ &= \frac{BT}{CT} \cdot \frac{BM}{CM} = \frac{BT}{CT} \cdot \frac{KA_1}{KA_2} = \frac{BT}{CT} \cdot \frac{AP}{AQ}. \end{aligned}
Proof completed.

Proof 1 (Alternative). An alternative proof that points B,C,MB, C, M, and TT are concyclic.
Since K,P,QK, P, Q, and TT are concyclic, by Ptolemy's Theorem, KTPQ=PKQT+QKPTKT \cdot PQ = PK \cdot QT + QK \cdot PT. Also, by the Law of Sines, PQsinPKQ=TQsinTKQ=PTsinPKT\frac{PQ}{\sin \angle PKQ} = \frac{TQ}{\sin \angle TKQ} = \frac{PT}{\sin \angle PKT}, hence
KT=PKQT+QKPTPQ=PKsinTKQ+QKsinPKTsinPKQ. KT = \frac{PK \cdot QT + QK \cdot PT}{PQ} = \frac{PK \cdot \sin \angle TKQ + QK \cdot \sin \angle PKT}{\sin \angle PKQ}.
Let ABC=B\angle ABC = \angle B and ACB=C\angle ACB = \angle C.
Noting that KP=2BKcosBKP = 2BK \cos \angle B and KQ=2CKcosCKQ = 2CK \cos \angle C, and utilizing the parallel relationships given in the problem, we obtain
KT=2BKcosBsinCAK+2CKcosCsinBAKsin(B+C)=2BKcosBKCKAsinC+2CKcosCKBKAsinBsin(B+C)=cosBsinC+cosCsinBsin(B+C)2KBKCKA=KBKCKM, \begin{aligned} KT &= \frac{2BK \cos \angle B \cdot \sin \angle CAK + 2CK \cos \angle C \cdot \sin \angle BAK}{\sin(\angle B + \angle C)} \\ &= \frac{2BK \cos \angle B \cdot \frac{KC}{KA} \sin \angle C + 2CK \cos \angle C \cdot \frac{KB}{KA} \sin \angle B}{\sin(\angle B + \angle C)} \\ &= \frac{\cos \angle B \cdot \sin \angle C + \cos \angle C \cdot \sin \angle B}{\sin(\angle B + \angle C)} \cdot \frac{2KB \cdot KC}{KA} = \frac{KB \cdot KC}{KM}, \end{aligned}
therefore KMKT=KBKCKM \cdot KT = KB \cdot KC. By the Power of a Point Theorem, it follows that points B,C,MB, C, M, and TT are concyclic. Proof completed.

Proof 2. Let's consider the circumcircle of ABC\triangle ABC as the unit circle in the complex plane. We use uppercase letters for points and the corresponding lowercase letters for their complex numbers.
Let XX be the intersection point of the line segment AKAK with the unit circle. Then
kˉ=a+xbcaxbc. \bar{k} = \frac{a + x - b - c}{ax - bc}.
Thus,
k=aˉ+xˉbˉcˉaˉxˉbˉcˉ=1a+1x1b1c1ax1bc=bc(x+a)ax(c+b)bcax=(axaccx)b+axcaxbc. k = \frac{\bar{a} + \bar{x} - \bar{b} - \bar{c}}{\bar{a}\bar{x} - \bar{b}\bar{c}} = \frac{\frac{1}{a} + \frac{1}{x} - \frac{1}{b} - \frac{1}{c}}{\frac{1}{ax} - \frac{1}{bc}} = \frac{bc(x + a) - ax(c + b)}{bc - ax} = \frac{(ax - ac - cx)b + axc}{ax - bc}.
Note the following geometric facts: the perpendicular bisector of KPKP passes through BB and is perpendicular to ABAB, the perpendicular bisector of KQKQ passes through CC and is perpendicular to ACAC. Therefore, the circumcenter of PQK\triangle PQK is the diametrically opposite point of AA on the unit circle, denoted as A1A_1. From AXA1=π2\angle AXA_1 = \frac{\pi}{2}, we know XX is the midpoint of TKTK.
Therefore,
k=1a+1x1b1c1ax1bc=bc(x+a)ax(c+b)bcax=(axaccx)b+axcaxbc. k = \frac{\frac{1}{a} + \frac{1}{x} - \frac{1}{b} - \frac{1}{c}}{\frac{1}{ax} - \frac{1}{bc}} = \frac{bc(x + a) - ax(c + b)}{bc - ax} = \frac{(ax - ac - cx)b + axc}{ax - bc}.

We observe the following geometric facts: The perpendicular bisector of the segment KPKP is the line passing through point BB and perpendicular to ABAB, and the perpendicular bisector of KQKQ is the line passing through CC and perpendicular to ACAC. Therefore, the circumcenter of PQK\triangle PQK is the diametrical opposite point of AA on the unit circle, denoted as A1A_1. Thus, according to AXA1=π2\angle AXA_1 = \frac{\pi}{2}, we know that XX is the midpoint of TKTK.
So
t=2xk=2x(axaccx)b+acxaxbc=2ax2(axac+cx)baxcaxbc,tb=2ax2(axac+cx)baxcaxb+b2caxbc=(bx)(bc+ac2ax)axbc,tc=2ax2(axac+cx)baxcaxc+bc2axbc=(cx)(ab+bc2ax)axbc. \begin{aligned} t &= 2x - k = 2x - \frac{(ax - ac - cx)b + acx}{ax - bc} = \frac{2ax^2 - (ax - ac + cx)b - axc}{ax - bc}, \\ t - b &= \frac{2ax^2 - (ax - ac + cx)b - axc - axb + b^2c}{ax - bc} = \frac{(b - x)(bc + ac - 2ax)}{ax - bc}, \\ t - c &= \frac{2ax^2 - (ax - ac + cx)b - axc - axc + bc^2}{ax - bc} = \frac{(c - x)(ab + bc - 2ax)}{ax - bc}. \end{aligned}
On the other hand, since QQ and KK are symmetric about the line CA1CA_1 (noting that a1=aa_1 = -a), we have
q+ac+a=k+ac+a=(kˉ+aˉ)aca+c=ackˉ+ca+c, \frac{q+a}{c+a} = \frac{\overline{k+a}}{c+a} = \frac{(\bar{k}+\bar{a})ac}{a+c} = \frac{ac\bar{k}+c}{a+c},
thus q=a+c+ackˉ=(ca)+ac(a+xbc)axbc. Hence, \text{thus } q = -a + c + ac\bar{k} = (c - a) + \frac{ac(a+x-b-c)}{ax-bc}. \text{ Hence,}
qc=a(axbc)+ac(a+xbc)axbc=a2x+a2c+acxac2axbc=a(ac)(cx)axbc,qa=(c2a)(axbc)+ac(a+xbc)axbc=abcbc22a2x+2acx+a2cac2axbc=(ac)(ac+bc2ax)axbc. \begin{aligned} q - c &= \frac{-a(ax - bc) + ac(a + x - b - c)}{ax - bc} = \frac{-a^2x + a^2c + acx - ac^2}{ax - bc} = \frac{a(a - c)(c - x)}{ax - bc}, \\ q - a &= \frac{(c - 2a)(ax - bc) + ac(a + x - b - c)}{ax - bc} = \frac{abc - bc^2 - 2a^2x + 2acx + a^2c - ac^2}{ax - bc} \\ &= \frac{(a - c)(ac + bc - 2ax)}{ax - bc}. \end{aligned}
Similarly, we have p=a+b+abkˉ=(ba)+ab(a+xbc)axbc. Thus, \text{Similarly, we have } p = -a + b + ab\bar{k} = (b - a) + \frac{ab(a+x-b-c)}{ax-bc}. \text{ Thus,}
pb=a(axbc)+ab(a+xbc)axbc=a2x+a2b+abxab2axbc=a(ab)(bx)axbc,pa=(b2a)(axbc)+ab(a+xbc)axbc=abcb2c2a2x+2abx+a2bab2axbc=(ab)(ab+bc2ax)axbc. \begin{aligned} p - b &= \frac{-a(ax - bc) + ab(a + x - b - c)}{ax - bc} = \frac{-a^2x + a^2b + abx - ab^2}{ax - bc} = \frac{a(a - b)(b - x)}{ax - bc}, \\ p - a &= \frac{(b - 2a)(ax - bc) + ab(a + x - b - c)}{ax - bc} = \frac{abc - b^2c - 2a^2x + 2abx + a^2b - ab^2}{ax - bc} \\ &= \frac{(a - b)(ab + bc - 2ax)}{ax - bc}. \end{aligned}
Therefore, we arrive at
tctbpbpa=qcqa, \frac{t-c}{t-b} \cdot \frac{p-b}{p-a} = \frac{q-c}{q-a},
and by taking the argument of both sides of this equation, we obtain conclusion (1); by taking the modulus, we obtain conclusion (2).

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 and solution reproduced as published; topic and difficulty added by this site.