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Geometry Difficulty 7.3 National Olympiad, round 2 Prove it United States

Let ABCDABCD be a quadrilateral, and let EE and FF be points on sides ADAD and BCBC, respectively, such that AE/ED=BF/FCAE/ED = BF/FC. Ray FEFE meets rays BABA and CDCD at SS and TT, respectively. Prove that the circumcircles of triangles SAESAE, SBFSBF, TCFTCF, and TDETDE pass through a common point.

Solutions — 2

Solution 1

Let PP be the second intersection of the circumcircles of triangles TCFTCF and TDETDE. Because the quadrilateral PEDTPEDT is cyclic, PET=PDT\angle PET = \angle PDT, or
PEF=PDC.() \angle PEF = \angle PDC. \quad (*)
Because the quadrilateral PFCTPFCT is cyclic,
PFE=PFT=PCT=PCD.() \angle PFE = \angle PFT = \angle PCT = \angle PCD. \quad (**)
By equations () and (*), it follows that triangle PEFPEF is similar to triangle PDCPDC. Hence FPE=CPD\angle FPE = \angle CPD and PF/PE=PC/PDPF/PE = PC/PD. Note also that FPC=FPE+EPC=CPD+EPC=EPD\angle FPC = \angle FPE + \angle EPC = \angle CPD + \angle EPC = \angle EPD. Thus, triangle EPDEPD is similar to triangle FPCFPC. Another way to say this is that there is a spiral similarity centered at PP that sends triangle PFEPFE to triangle PCDPCD, which implies that there is also a spiral similarity, centered at PP, that sends triangle PFCPFC to triangle PEDPED, and vice versa. In terms of complex numbers, this amounts to saying that
DPEP=CPFP    EPFP=DPCP. \frac{D-P}{E-P} = \frac{C-P}{F-P} \implies \frac{E-P}{F-P} = \frac{D-P}{C-P}.
Figure 1
Because AE/ED=BF/FCAE/ED = BF/FC, points AA and BB are obtained by extending corresponding segments of two similar triangles PEDPED and PFCPFC, namely, DEDE and CFCF, by the identical proportion. We conclude that triangle PDAPDA is similar to triangle PCBPCB, implying that triangle PAEPAE is similar to triangle PBFPBF. Therefore, as shown before, we can establish the similarity between triangles PBAPBA and PFEPFE, implying that
PBS=PBA=PFE=PFSandPAB=PEF. \angle PBS = \angle PBA = \angle PFE = \angle PFS \quad \text{and} \quad \angle PAB = \angle PEF.

Solution 2

We will give a solution using complex coordinates. The first step is the following lemma.

Lemma Suppose ss and tt are real numbers and xx, yy and zz are complex. The circle in the complex plane passing through xx, x+tyx + ty and x+(s+t)zx + (s + t)z also passes through the point x+syz/(yz)x + syz/(y - z), independent of tt.

Proof: Four points z1,z2,z3z_1, z_2, z_3 and z4z_4 in the complex plane lie on a circle if and only if the cross-ratio
cr(z1,z2,z3,z4)=(z1z3)(z2z4)(z1z4)(z2z3) cr(z_1, z_2, z_3, z_4) = \frac{(z_1 - z_3)(z_2 - z_4)}{(z_1 - z_4)(z_2 - z_3)}
is real. Since we compute
cr(x,x+ty,x+(s+t)z,x+syz/(yz))=s+ts cr(x, x + ty, x + (s + t)z, x + syz/(y - z)) = \frac{s + t}{s}
the given points are on a circle.

Lay down complex coordinates with S=0S = 0 and EE and FF on the positive real axis. Then there are real r1,r2r_1, r_2 and RR with B=r1AB = r_1A, F=r2EF = r_2E and D=E+R(AE)D = E + R(A - E) and hence AE/ED=BF/FCAE/ED = BF/FC gives
C=F+R(BF)=r2(1R)E+r1RA. C = F + R(B - F) = r_2(1 - R)E + r_1RA.
The line CDCD consists of all points of the form sC+(1s)DsC + (1-s)D for real ss. Since TT lies on this line and has zero imaginary part, we see from Im(sC+(1s)D)=(sr1R+(1s)R)Im(A)\text{Im}(sC + (1-s)D) = (sr_1R + (1-s)R)\text{Im}(A) that it corresponds to s=1/(r11)s = -1/(r_1 - 1). Thus
T=r1DCr11=(r2r1)(R1)Er11. T = \frac{r_1D - C}{r_1 - 1} = \frac{(r_2 - r_1)(R - 1)E}{r_1 - 1}.
Apply the lemma with x=Ex = E, y=AEy = A - E, z=(r2r1)E/(r11)z = (r_2 - r_1)E/(r_1 - 1), and s=(r21)(r1r2)s = (r_2 - 1)(r_1 - r_2). Setting t=1t = 1 gives
(x,x+y,x+(s+1)z)=(E,A,S=0) (x, x + y, x + (s + 1)z) = (E, A, S = 0)
and setting t=Rt = R gives
(x,x+Ry,x+(s+R)z)=(E,D,T). (x, x + Ry, x + (s + R)z) = (E, D, T).
Therefore the circumcircles to SAESAE and TDETDE meet at
x+syzyz=AE(r1r2)(1r1)E(1r2)A=AFBEA+FBE. x + \frac{syz}{y - z} = \frac{AE(r_1 - r_2)}{(1 - r_1)E - (1 - r_2)A} = \frac{AF - BE}{A + F - B - E}.
This last expression is invariant under simultaneously interchanging AA and BB and interchanging EE and FF. Therefore it is also the intersection of the circumcircles of SBFSBF and TCFTCF.

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