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Geometry Difficulty 6.9 National Olympiad Prove it India

Problem:

Betal marks 20212021 points on the plane such that no three are collinear, and draws all possible line segments joining these. He then chooses any 10111011 of these line segments, and marks their midpoints. Finally, he chooses a line segment whose midpoint is not marked yet, and challenges Vikram to construct its midpoint using only a straightedge. Can Vikram always complete this challenge?

Note: A straightedge is an infinitely long ruler without markings, which can only be used to draw the line joining any two given distinct points.

Solutions — 2

Solution 1

Solution:

The answer is 'yes'. To prove this, we will first prove two lemmas:

Lemma 1 Given any two points A,BA, B, their midpoint MM, and any point CC, Vikram can draw a line parallel to ABA B through CC.

Proof. If CC is on line ABA B we are already done. If not, extend BCB C to XX as shown, draw P=ACXMP = A C \cap X M, and then draw D=BPAXD = B P \cap A X. We claim CDC D is the desired line. Indeed, using Ceva's theorem on triangle ABXA B X and the fact AM=MBA M = M B, we see that
AMMBBCCXXDDA=1XCCB=XDDA \frac{A M}{M B} \cdot \frac{B C}{C X} \cdot \frac{X D}{D A} = 1 \Longrightarrow \frac{X C}{C B} = \frac{X D}{D A}
This means CDABC D \parallel A B.

Figure 1

Lemma 2 Given two non-parallel segments AB,BCA B, B C and their midpoints M,NM, N, Vikram can draw the midpoint of any other segment XYX Y.

Proof. Assume first XYX Y is not parallel to ABA B or BCB C. Using lemma 1, draw lines 1\ell_1 and 2\ell_2 through XX parallel to ABA B and BCB C respectively, and similarly draw m1m_1 and m2m_2 through YY parallel to ABA B and BCB C respectively. If we draw P=1m2P = \ell_1 \cap m_2 and Q=2m1Q = \ell_2 \cap m_1, then XPYQX P Y Q is a parallelogram, so intersecting PQP Q and XYX Y gives the midpoint of XYX Y.

As for the remaining case, one can draw ACA C and construct the midpoint PP of ACA C by the construction described above. Since XYX Y can be parallel to at most one of the sides AB,BCA B, B C and ACA C, we can pick the two non-parallel sides, and use the above construction to draw the midpoint of XYX Y.

Now for the main problem, note that if no two of the 10111011 chosen segments share an endpoint, then we have at least 21011=20222 \cdot 1011 = 2022 distinct endpoints, a contradiction. Thus there must be two segments ABA B and BCB C which have their midpoints marked. Since no three of the chosen 20212021 points were collinear, ABA B and BCB C are not parallel, so using lemma 2, Vikram can construct the midpoint of any other segment, in particular, the segment chosen by Betal.

Solution 2

Solution:

As in the previous solution, note that there exist ABA B and ACA C whose midpoints CC^{\prime} and BB^{\prime} are marked. Using the straightedge, Vikram can draw the two medians ACA C^{\prime} and ABA B^{\prime} and obtain their intersection, the centroid GG of ABC\triangle A B C. Now intersecting AGA G with BCB C gives AA^{\prime}, the midpoint of BCB C.

Lemma Given a point PP not on AB,ACA B, A C, Vikram can draw the midpoint of PAP A.

Proof. If PBACP B \parallel A C and PCABP C \parallel A B, then PBACP B A C is a parallelogram, in which case AA^{\prime} constructed above is the midpoint of PAP A. Without loss of generality, we may assume PBACP B \nparallel A C.

Figure 2

Using the straightedge, one can mark the points D=PBACD = P B \cap A C and PBAC=DP B \cap A^{\prime} C^{\prime} = D^{\prime}. Since CAACC A \parallel A^{\prime} C^{\prime}, we have
BDDD=BCCA=1 \frac{B D^{\prime}}{D^{\prime} D} = \frac{B C^{\prime}}{C^{\prime} A} = 1
so DD^{\prime} is the midpoint of BDB D. Now in ABD\triangle A B D, two midpoints CC^{\prime} and DD^{\prime} are known, so the midpoint QQ^{\prime} of ADA D can be constructed using the centroid construction outlined before. Let P=CQPAP^{\prime} = C^{\prime} Q^{\prime} \cap P A; this exists as CQBPAPC^{\prime} Q^{\prime} \parallel B P \nparallel A P. As before, CPBPC^{\prime} P^{\prime} \parallel B P, so
APPP=ACCB=1 \frac{A P^{\prime}}{P^{\prime} P} = \frac{A C^{\prime}}{C^{\prime} B} = 1
which means PP^{\prime} is the desired midpoint of PAP A.

Now suppose we need to find the midpoint of PQP Q. If P,QP, Q are different points from AA, then one can draw the midpoints of APA P and AQA Q using the lemma. Then by using the centroid of APQ\triangle A P Q, one can find the midpoint of PQP Q as we did for BCB C. If PP or QQ is AA, the above lemma immediately yields the required midpoint.

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