Maths Olympiad Prep

Library / Belarusian Mathematical Olympiad /175 of 208

Belarusian Mathematical Olympiad

Geometry Difficulty 6.9 National Olympiad Prove it Belarus

Let DD, EE, FF denote the tangent points of the incircle of ABCABC with the sides BCBC, ACAC, ABAB, respectively. Let MM be the midpoint of the segment EFEF. Let LL be the intersection point of the circle passing through DD, MM, FF and the segment ABAB, KK be the intersection point of the circle passing through DD, MM, EE and the segment ACAC.
Prove that the circle passing through AA, KK, LL touches the line BCBC.
(V. Voinov)

Solution

Let I be the incenter of the triangle ABC. Let Γ\Gamma denote the circle passing through AA, KK, LL. We have
∠AKD=∠EKD=180∘−∠DME=∠DMF=180∘−∠FLD==180∘−∠ALD. \begin{align*} \angle AKD = \angle EKD &= 180^\circ - \angle DME = \angle DMF = 180^\circ - \angle FLD = \\ &= 180^\circ - \angle ALD. \end{align*}
Therefore, DD lies on Γ\Gamma. So, to prove that Γ\Gamma touches BCBC it suffices to show that ∠DAL=∠BDL\angle DAL = \angle BDL.

Figure 1

Since AE=AFAE = AF and EM=MFEM = MF, it follows that AM⊥EFAM \perp EF and AMAM is the bisector of the angle EAFEAF, i.e., the points AA, MM, II lie on the same line.
We have
∠BDL=90∘−∠IDL=90∘−(∠IDM+∠MDL)==(90∘−∠MDL)−∠IDM=[∠MDL=180∘−∠MFL=∠MFA]==(90∘−∠MFA)−∠IDM=∠FAM−∠IDM==∠DAL+∠DAM−∠IDM. \begin{align*} \angle BDL &= 90^\circ - \angle IDL = 90^\circ - (\angle IDM + \angle MDL) = \\ &= (90^\circ - \angle MDL) - \angle IDM = [\angle MDL = 180^\circ - \angle MFL = \angle MFA] = \\ &= (90^\circ - \angle MFA) - \angle IDM = \angle FAM - \angle IDM = \\ &= \angle DAL + \angle DAM - \angle IDM. \tag{1} \end{align*}

Since the triangle AFIAFI is the right-angled triangle and FM⊥AIFM \perp AI, we have IF2=IM⋅IAIF^2 = IM \cdot IA. Since IF=IDIF = ID, we obtain ID2=IM⋅IAID^2 = IM \cdot IA. By the Power of a Point Theorem, the circle passing through the points AA, MM, DD touches the line IDID at DD, then ∠IDM=∠DAM\angle IDM = \angle DAM. From (1) it follows that ∠DAL=∠BDL\angle DAL = \angle BDL as required.

Want a route through all this instead of an archive? The track puts 2,604 problems in a working order, from Junior Challenge level to the IMO shortlist.

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