Maths Olympiad Prep

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Geometry Difficulty 5.1 AIME, harder Prove it Ukraine

Bisector of the angle ∠ABC\angle ABC of the triangle ABCABC intersects the circumcircle of triangle ABCABC at the point KK. Point NN belongs to the segment ABAB and NK⊥ABNK \perp AB. Let PP be a midpoint of the segment NBNB. Consider the line through PP that is parallel to BCBC and intersects line BKBK at the point TT. Prove that the line NTNT passes through the midpoint of the segment ACAC.

(Nagel Igor)

Figure 1

Solution

Let M=NT∩ACM = NT \cap AC (fig. 23). We note that since BKBK is a bisector of ABCABC then ∠KBC=∠KBA=α\angle KBC = \angle KBA = \alpha, and since PTPT is parallel to BCBC then ∠KBC=∠PTB\angle KBC = \angle PTB. Thus, PT=PB=PNPT = PB = PN. It follows that △BNT\triangle BNT is right-angled with hypotenuse BNBN. Since triangle △BNK\triangle BNK is also right-angled then ∠KNT=α\angle KNT = \alpha. Moreover, we get ∠KAC=∠KBC=α\angle KAC = \angle KBC = \alpha. Thus, quadrilateral KANMKANM is cyclic. Since ∠ANK=90∘\angle ANK = 90^\circ, then AKAK is a diameter of the circumcircle of the KANMKANM. Hence, ∠AMK=90∘\angle AMK = 90^\circ. Since △AKC\triangle AKC is isosceles we get that altitude KMKM also is a median. Hence, AM=MCAM = MC.

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