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Geometry Difficulty 9.0 Shortlist Prove it IMO

Let ABCABC be an equilateral triangle. Points A1A_1, B1B_1, C1C_1 lie inside triangle ABCABC such that triangle A1B1C1A_1B_1C_1 is scalene, BA1=A1CBA_1 = A_1C, CB1=B1ACB_1 = B_1A, AC1=C1BAC_1 = C_1B and
BA1C+CB1A+AC1B=480. \angle BA_1C + \angle CB_1A + \angle AC_1B = 480^\circ.
Lines BC1BC_1 and CB1CB_1 intersect at A2A_2; lines CA1CA_1 and AC1AC_1 intersect at B2B_2; and lines AB1AB_1 and BA1BA_1 intersect at C2C_2.
Prove that the circumcircles of triangles AA1A2AA_1A_2, BB1B2BB_1B_2, CC1C2CC_1C_2 have two common points.

Solution

Let δA\delta_A, δB\delta_B, δC\delta_C be the circumcircles of AA1A2\triangle AA_1A_2, BB1B2\triangle BB_1B_2, CC1C2\triangle CC_1C_2. The general strategy of the solution is to find two different points having equal power with respect to δA\delta_A, δB\delta_B, δC\delta_C.

Claim. A1A_1 is the circumcentre of A2BCA_2BC and cyclic variations.

Proof. Since A1A_1 lies on the perpendicular bisector of BCBC and inside BA2C\triangle BA_2C, it suffices to prove BA1C=2BA2C\angle BA_1C = 2 \angle BA_2C. This follows from
BA2C=A2BA+BAC+ACA2=12((180AC1B)+(180CB1A))+60=24012(480BA1C)=12BA1C \begin{aligned} \angle BA_2C &= \angle A_2BA + \angle BAC + \angle ACA_2 \\ &= \frac{1}{2}\left((180^\circ - \angle AC_1B) + (180^\circ - \angle CB_1A)\right) + 60^\circ \\ &= 240^\circ - \frac{1}{2}(480^\circ - \angle BA_1C) \\ &= \frac{1}{2} \angle BA_1C \end{aligned}
\square

Figure 1

The circumcentres above give
B1B2C1=B1B2A=B2AB1=C1AC2=AC2C1=B1C2C1 \angle B_1B_2C_1 = \angle B_1B_2A = \angle B_2AB_1 = \angle C_1AC_2 = \angle AC_2C_1 = \angle B_1C_2C_1
and so B1C1B2C2B_1C_1B_2C_2 is cyclic. Likewise C1A1C2A2C_1A_1C_2A_2 and A1B1A2B2A_1B_1A_2B_2 are cyclic. Note that hexagon A1B2C1A2B1C2A_1B_2C_1A_2B_1C_2 is not cyclic since
C2A1B2+B2C1A2+A2B1C2=480360. \angle C_2A_1B_2 + \angle B_2C_1A_2 + \angle A_2B_1C_2 = 480^\circ \neq 360^\circ.
Thus we can apply radical axis theorem to the three circles to show that A1A2A_1A_2, B1B2B_1B_2, C1C2C_1C_2 concur at a point XX and this point has equal power with respect to δA\delta_A, δB\delta_B, δC\delta_C.

Let the circumcircle of A2BC\triangle A_2BC meet δA\delta_A at A3A2A_3 \neq A_2. Define B3B_3 and C3C_3 similarly.

Claim. BCB3C3BCB_3C_3 cyclic.

Proof. Using directed angles
BC3C=BC3C2+̸C2C3C=BAC2+̸C2C1C=90+(C1C,AC2)+̸C2C1C=90+C1C2B1. \begin{align*} \npreceq BC_3C &= \measuredangle BC_3C_2 + \not C_2C_3C \\ &= \measuredangle BAC_2 + \not C_2C_1C \\ &= 90^\circ + \nmid(C_1C, AC_2) + \not C_2C_1C \tag{1} \\ &= 90^\circ + \nmid C_1C_2B_1. \end{align*}
Similarly CB3B=90+B1B2C1\nsucceq CB_3B = 90^\circ + \measuredangle B_1B_2C_1. Hence, using B1C1B2C2B_1C_1B_2C_2 cyclic
BB 3C = 90 + C 1B 2B 1 = 90 + C 1C 2B 1 = BC 3C\text{BB 3C = 90 + C 1B 2B 1 = 90 + C 1C 2B 1 = BC 3C}
as required. \square

Figure 2

Similarly CAC3A3CAC_3A_3 and ABA3B3ABA_3B_3 are cyclic. AC3BA3CB3AC_3BA_3CB_3 is not cyclic because then AB2CB3AB_2CB_3 cyclic would mean B2B_2 lies on ABC\odot ABC which is impossible since B2B_2 lies inside ABC\triangle ABC. Thus we can apply radical axis theorem to the three circles to get AA3AA_3, BB3BB_3, CC3CC_3 concur at a point YY which has equal power with respect to δA\delta_A, δB\delta_B, δC\delta_C.

We now make some technical observations before finishing.
- Let OO be the centre of ABC\triangle ABC. We have that
BA1C=480CB1AAC1B>480180180=120. \angle BA_1C = 480^\circ - \angle CB_1A - \angle AC_1B > 480^\circ - 180^\circ - 180^\circ = 120^\circ.
so A1A_1 lies inside BOC\triangle BOC. We have similar results for B1B_1, C1C_1 and thus BA1C\triangle BA_1C, CB1A\triangle CB_1A, AC1B\triangle AC_1B have disjoint interiors. It follows that A1B2C1A2B1C2A_1B_2C_1A_2B_1C_2 is a convex hexagon thus XX lies on segment A1A2A_1A_2 and therefore is inside δA\delta_A.
- Since A1A_1 is the centre of A2BCA_2BC we have that A1A2=A1A3A_1A_2 = A_1A_3 so, from cyclic quadrilateral AA2A1A3AA_2A_1A_3 we get that lines AA2AA_2 and AA3AYAA_3 \equiv AY are reflections in line AA1AA_1. As XX lies on segment A1A2A_1A_2, the only way XYX \equiv Y is if A1A_1 and A2A_2 both lie on the perpendicular bisector of BCBC. But this forces B1B_1 and C1C_1 to also be reflections in this line meaning A1B1=A1C1A_1B_1 = A_1C_1 contradicting the scalene condition.

Summarising, we have distinct points X,YX, Y with equal power with respect to δA\delta_A, δB\delta_B, δC\delta_C thus these circles have a common radical axis. As XX lies inside δA\delta_A (and similarly δB\delta_B, δC\delta_C), this radical axis intersects the circles at two points and so δA\delta_A, δB\delta_B, δC\delta_C have two points in common.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.