Bisector of the angle ∠ABC of the triangle ABC intersects the circumcircle of triangle ABC at the point K. Point N belongs to the segment AB and NK⊥AB. Let P be a midpoint of the segment NB. Consider the line through P that is parallel to BC and intersects line BK at the point T. Prove that the line NT passes through the midpoint of the segment AC.
(Nagel Igor)
Solution
Let M=NT∩AC (fig. 23). We note that since BK is a bisector of ABC then ∠KBC=∠KBA=α, and since PT is parallel to BC then ∠KBC=∠PTB. Thus, PT=PB=PN. It follows that △BNT is right-angled with hypotenuse BN. Since triangle △BNK is also right-angled then ∠KNT=α. Moreover, we get ∠KAC=∠KBC=α. Thus, quadrilateral KANM is cyclic. Since ∠ANK=90∘, then AK is a diameter of the circumcircle of the KANM. Hence, ∠AMK=90∘. Since △AKC is isosceles we get that altitude KM also is a median. Hence, AM=MC.
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