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Geometry Difficulty 6.6 National olympiad Prove it Croatia

Let ABCABC be an acute-angled triangle such that AB>AC|AB| > |AC|, and let DD, EE and FF be the feet of its altitudes from the vertices AA, BB and CC, respectively. The lines EFEF and BCBC intersect at the point PP. The line passing through DD parallel to EFEF intersects the lines ACAC and ABAB at the points QQ and RR, respectively.

If NN is a point on the segment BC\overline{BC} such that NQP+NRP<180\angle NQP + \angle NRP < 180^\circ, prove that BN>CN|BN| > |CN|.

(South Africa 2010)

Solution

The triangle ABCABC is acute-angled, so the points DD, EE and FF lie on the sides BC\overline{BC}, CA\overline{CA} and AB\overline{AB}, respectively. Since AB>AC|AB| > |AC|, we have BD>CD|BD| > |CD|.

Furthermore, from ACB<ABC\angle ACB < \angle ABC it follows that CC lies between BB and PP. Also, CC lies between AA and QQ, while RR lies on the segment BF\overline{BF}.

Figure 1

Let HH be the orthocentre of the triangle ABCABC, and let MM be the midpoint of its side BC\overline{BC}. The lines DRDR and EFEF are parallel, hence DRF=EFA\angle DRF = \angle EFA. The quadrilateral AFHEAFHE is cyclic (AFH=AEH=90\angle AFH = \angle AEH = 90^\circ), so we have EFA=EHA\angle EFA = \angle EHA. The quadrilateral BDHFBDHF is cyclic as well (BDH=BFH=90\angle BDH = \angle BFH = 90^\circ), hence EHA=DHB=DFB=DFR\angle EHA = \angle DHB = \angle DFB = \angle DFR. Now from DRF=DFR\angle DRF = \angle DFR it follows that DR=DF|DR| = |DF|.

Similarly, the lines DQDQ and FEFE are parallel, hence DQE=FEA\angle DQE = \angle FEA. The quadrilateral AFHEAFHE is cyclic, so we have FEA=FHA\angle FEA = \angle FHA. The quadrilateral CEHDCEHD is cyclic as well (CEH=CDH=90\angle CEH = \angle CDH = 90^\circ), hence FHA=DHC=DEC=DEQ\angle FHA = \angle DHC = \angle DEC = \angle DEQ. Now from DQE=DEQ\angle DQE = \angle DEQ it follows that DQ=DE|DQ| = |DE|.

The points MM, FF, EE and DD lie on the same circle (called the nine-point circle or the Feuerbach's circle), hence FMD=180FED=PED\angle FMD = 180^\circ - \angle FED = \angle PED. Also, FDH=FBH=ABE=90CAB=ACF=ECH=EDH\angle FDH = \angle FBH = \angle ABE = 90^\circ - \angle CAB = \angle ACF = \angle ECH = \angle EDH.

Therefore, FDM=90FDH=90EDH=PDE\angle FDM = 90^\circ - \angle FDH = 90^\circ - \angle EDH = \angle PDE, so the triangles FDMFDM and PDEPDE are similar. Now we conclude that
DFDM=DPDE, \frac{|DF|}{|DM|} = \frac{|DP|}{|DE|},
from which it follows that DFDE=DPDM|DF| \cdot |DE| = |DP| \cdot |DM|, i.e.
DRDQ=DPDM, |DR| \cdot |DQ| = |DP| \cdot |DM|,
meaning that the quadrilateral PQMRPQMR is cyclic.

Let kk be its circumscribed circle. Since NQP+NRP<180\angle NQP + \angle NRP < 180^\circ, NN lies inside the circle kk, i.e. on the segment CM\overline{CM}, and NMN \neq M holds. Since MM is the midpoint of the segment BC\overline{BC}, we finally get BN>CN|BN| > |CN|.

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Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.