Maths Olympiad Prep

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Geometry Difficulty 6.6 National Olympiad Prove it JBMO

Problem:
Let HH be the orthocentre of an acute triangle ABCA B C with BC>ACB C > A C, inscribed in a circle Γ\Gamma. The circle with centre CC and radius CBC B intersects Γ\Gamma at the point DD, which is on the arc ABA B not containing CC. The circle with centre CC and radius CAC A intersects the segment CDC D at the point KK. The line parallel to BDB D through KK, intersects ABA B at point LL. If MM is the midpoint of ABA B and NN is the foot of the perpendicular from HH to CLC L, prove that the line MNM N bisects the segment CHC H.

Solution

Solution:
We use standard notation for the angles of triangle ABCA B C. Let PP be the midpoint of CHC H and OO the centre of Γ\Gamma. As
α=BAC=BDC=DKL \alpha = \angle B A C = \angle B D C = \angle D K L
the quadrilateral ACKLA C K L is cyclic. From the relation CB=CDC B = C D we get BCD=1802α\angle B C D = 180^\circ - 2 \alpha, so
ACK=γ+2α180 \angle A C K = \gamma + 2 \alpha - 180^\circ
where γ=ACB\gamma = \angle A C B. From the relation CK=CAC K = C A we get
ALC=AKC=180αγ2 \angle A L C = \angle A K C = 180^\circ - \alpha - \frac{\gamma}{2}
and thus from the triangle ACLA C L we obtain
ACL=180αALC=γ2 \angle A C L = 180^\circ - \alpha - \angle A L C = \frac{\gamma}{2}
which means that CLC L is the angle bisector of ACB\angle A C B, thus ACL=BCL\angle A C L = \angle B C L. Moreover, from the fact that CHABC H \perp A B and the isosceles triangle BOCB O C has BOC=2α\angle B O C = 2 \alpha, we get ACH=BCO=90α\angle A C H = \angle B C O = 90^\circ - \alpha. It follows that,
NPH=2NCH=OCH \angle N P H = 2 \angle N C H = \angle O C H
Figure 1
On the other hand, it is known that 2CP=CH=2OM2 C P = C H = 2 O M and CPOMC P \parallel O M, so CPMOC P M O is a parallelogram and
MPH=OCH \angle M P H = \angle O C H
Now from (3) and (4) we obtain that
MPH=NPH, \angle M P H = \angle N P H,
which means that the points M,N,PM, N, P are collinear.

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