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Geometry Difficulty 6.2 National Olympiad Prove it Taiwan

Let ABC\triangle ABC be an acute-angled triangle, with BC\angle B \neq \angle C. Let MM be the midpoint of side BCBC, and EE, FF be the feet of the altitudes from BB, CC, respectively. Denote by KK, LL the midpoints of segments MEME, MFMF, respectively. Suppose TT is a point on the line KLKL such that ATBCAT \parallel BC.
Prove that TA=TMTA = TM.

Solution

Without loss of generality, assume AB>ACAB > AC. Construct the circumcircle ω\omega of AEF\triangle AEF.

Lemma 1. The line MEME, the line MFMF, and the line ATAT are all tangent to circle ω\omega.

Figure 1

Proof. Note that EE, FF both lie on the circle with diameter BCBC, whose center is MM. Hence MC=ME=MF=MBMC = ME = MF = MB. Therefore
FEM=FEB+BEM=FCB+MBE=(90CBF)+(90ECB)=180CBAACB=FAE. \begin{aligned} \angle FEM &= \angle FEB + \angle BEM = \angle FCB + \angle MBE \\ &= (90^\circ - \angle CBF) + (90^\circ - \angle ECB) \\ &= 180^\circ - \angle CBA - \angle ACB = \angle FAE. \end{aligned}
So by the tangent-chord angle property, MEME is tangent to ω\omega. By symmetry, MFMF is also tangent to ω\omega.

Moreover, TAB=CBA=180BEC=AEF\angle TAB = \angle CBA = 180^\circ - \angle BEC = \angle AEF, so TATA is also tangent to ω\omega. ♡

Now consider the circle ω\omega, together with the circle γ\gamma centered at MM with radius 00. Since KE2=KM2KE^2 = KM^2, and KEKE is tangent to ω\omega while KMKM is tangent to γ\gamma, KK lies on the radical axis of ω\omega and γ\gamma. Similarly, so does LL. In other words, the line KLKL is exactly the radical axis of ω\omega and γ\gamma, and TT lies on this radical axis. Hence, considering the power of TT with respect to these two circles: TA2=TM2TA^2 = TM^2, which gives TA=TMTA = TM. This completes the proof.

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from en; metadata (topic, difficulty) added by this project.