Maths Olympiad Prep

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Geometry Difficulty 6.6 National Olympiad Prove it United States

Problem:

In an acute triangle ABCA B C let K,LK, L, and MM be the midpoints of sides AB,BCA B, B C, and CAC A, respectively. From each of K,LK, L, and MM drop two perpendiculars to the other two sides of the triangle; e.g., drop perpendiculars from KK to sides BCB C and CAC A, etc. The resulting 6 perpendiculars intersect at points Q,SQ, S, and TT as in the figure to form a hexagon KQLSMTK Q L S M T inside triangle ABCA B C. Prove that the area of the hexagon KQLSMTK Q L S M T is half of the area of the original triangle ABCA B C.

Figure 1

Solution

Solution:

Construct segments KL,LMK L, L M and MKM K. Next, construct the three altitudes of triangle KLMK L M that meet in its orthocenter, OO. Since KLK L connects the two midpoints of ABA B and BCB C we know that KLABK L \| A B it is easy to see that MTKOLQM T\|K O\| L Q since those lines are perpendicular to a pair of parallel lines. Similarly, KTMOLSK T\|M O\| L S and KQLOMSK Q\|L O\| M S.

Note that triangle KLMK L M divides the original triangle ABCA B C into four congruent parts, all similar to the original triangle (and thus all acute), so the perpendiculars we dropped from K,LK, L, and MM are just altitudes of the smaller triangles. Since all the smaller triangles are acute, all the altitudes will meet inside the respective triangles.

Because of all the parallel lines, we know that OLSM,OMTKO L S M, O M T K and OKQLO K Q L are all parallelograms having diagonals ML,KMM L, K M, and LKL K, respectively. The diagonal of a parallelogram divides it into two congruent triangles, so triangle LSML S M is congruent to triangle MOLM O L, triangle MTKM T K is congruent to triangle KOMK O M, and triangle KQLK Q L is congruent to triangle LOKL O K. If we consider the hexagon KQLSMTK Q L S M T to be composed of triangle KLMK L M and the three outer pieces, we can see that triangle KLMK L M is composed of three smaller triangles that are congruent to the corresponding outer pieces, so the area of the hexagon is twice the area of triangle KLMK L M.

But triangle KLMK L M connects the midpoints of the edges of triangle ABCA B C so each of its sides is half the length of the corresponding side of triangle ABCA B C, so triangle KLMK L M is similar to triangle ABCA B C, but with 1/41 / 4 the area. We previously showed that the area of KQLSMTK Q L S M T is twice the area of triangle KLMK L M, so the area of the hexagon is 1/21 / 2 the area of triangle ABCA B C.

Figure 2

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