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Geometry Difficulty 6.7 National Olympiad Prove it Croatia

Let MM be a point in the interior of the triangle ABCABC. The line AMAM intersects the circumcircle of the triangle MBCMBC once more at DD, the line BMBM intersects the circumcircle of the triangle MCAMCA once more at EE, and the line CMCM intersects the circumcircle of the triangle MABMAB once more at FF. Prove the inequality
ADMD+BEME+CFMF92.(Tajikistan 2014) \frac{|AD|}{|MD|} + \frac{|BE|}{|ME|} + \frac{|CF|}{|MF|} \ge \frac{9}{2}. \quad (\text{Tajikistan 2014})

Solution

It suffices to show that
AMMD+BMME+CMMF32. \frac{|AM|}{|MD|} + \frac{|BM|}{|ME|} + \frac{|CM|}{|MF|} \ge \frac{3}{2}.
Quadrilaterals MBDCMBDC and MCEAMCEA are cyclic, so
BCD=BMD=EMA=ECA. \angle BCD = \angle BMD = \angle EMA = \angle ECA.
Also, we have
DBC=DMC=180CMA=CEA. \angle DBC = \angle DMC = 180^\circ - \angle CMA = \angle CEA.
Hence we conclude that the triangles BDC and EAC are similar. Analogously, we prove that the triangle BAF is similar to them.

Figure 1

By Ptolemy's theorem for quadrilaterals MBDC, MCEA and MAFB, and by using the ratios from the similarity of BDC, EAC and BAF, we have:
MD=BMCDBC+CMDBBC,ME=CMAECA+AMECCA=CMDBCD+AMBCCD,MF=AMBFAB+BMFAAB=AMBCDB+BMCDDB, \begin{align*} |MD| &= |BM| \cdot \frac{|CD|}{|BC|} + |CM| \cdot \frac{|DB|}{|BC|}, \\ |ME| &= |CM| \cdot \frac{|AE|}{|CA|} + |AM| \cdot \frac{|EC|}{|CA|} = |CM| \cdot \frac{|DB|}{|CD|} + |AM| \cdot \frac{|BC|}{|CD|}, \\ |MF| &= |AM| \cdot \frac{|BF|}{|AB|} + |BM| \cdot \frac{|FA|}{|AB|} = |AM| \cdot \frac{|BC|}{|DB|} + |BM| \cdot \frac{|CD|}{|DB|}, \end{align*}
from where it follows that
AMMD+BMME+CMMF=AMBCBMCD+CMDB+BMCDCMDB+AMBC+CMDBAMBC+BMCD \frac{|AM|}{|MD|} + \frac{|BM|}{|ME|} + \frac{|CM|}{|MF|} = \frac{|AM| \cdot |BC|}{|BM| \cdot |CD| + |CM| \cdot |DB|} \\ + \frac{|BM| \cdot |CD|}{|CM| \cdot |DB| + |AM| \cdot |BC|} \\ + \frac{|CM| \cdot |DB|}{|AM| \cdot |BC| + |BM| \cdot |CD|}
Let us denote x=AMBCx = |AM| \cdot |BC|, y=BMCDy = |BM| \cdot |CD|, z=CMDBz = |CM| \cdot |DB|. We need to prove the inequality
xy+z+yz+x+zx+y32. \frac{x}{y+z} + \frac{y}{z+x} + \frac{z}{x+y} \geq \frac{3}{2}.
This is Nesbitt’s famous inequality, so the proof is finished.

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