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Geometry Difficulty 8.3 Shortlist Prove it Netherlands

A triangle ABCABC has the property that AB+AC=3BC|AB| + |AC| = 3|BC|. Let TT be the point on line segment ACAC satisfying AC=4AT|AC| = 4|AT|. Let KK and LL be points on the interior of line segments ABAB and ACAC, respectively, such that KLBCKL \parallel BC, and KLKL is tangent to the incircle of ABC\triangle ABC. Let SS be the intersection of BTBT and KLKL. Determine the ratio SLKL\frac{|SL|}{|KL|}.

Solution

Denote the radius of the incircle of ABC\triangle ABC by rr. Then the area of triangle ABCABC is
12ABr+12BCr+12AC=12r(3BC+BC)=2rBC. \frac{1}{2}|AB| \cdot r + \frac{1}{2}|BC| \cdot r + \frac{1}{2}|AC| = \frac{1}{2}r \cdot (3|BC| + |BC|) = 2r|BC|.
On the other hand, the area of ABCABC equals 12hBC\frac{1}{2}h|BC|, where hh is the altitude from AA. Hence, h=4rh = 4r. Because the distance from KLKL to BCBC is exactly 2r2r, the distance from AA to KLKL is also 2r2r. Triangles AKLAKL and ABCABC are similar, because KLBCKL \parallel BC, and the altitudes from AA have lengths 2r2r and 4r4r, respectively, giving a multiplication factor of exactly 2. Hence, KK is the midpoint ABAB, and LL is the midpoint of ACAC.
For the point TT, we have AC=4AT|AC| = 4|AT|, hence AT=14AC=12AL|AT| = \frac{1}{4}|AC| = \frac{1}{2}|AL|, hence TT is the midpoint of ALAL. Now consider triangle ABLABL. In this triangle, the segment BTBT is a median, because TT is the midpoint of ALAL. Also LKLK is a median as KK is the midpoint ABAB. Their intersection point SS is the centroid, from which we get that SLKL=23\frac{|SL|}{|KL|} = \frac{2}{3}.
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