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Geometry Difficulty 6.1 National olympiad Prove it Ukraine

What is the maximum number of points that can be placed on the plane so that there are exactly 20122012 straight lines, which pass through at least two of them?

(Danylo Mysak)

Figure 1
Fig. 24

Figure 2
Fig. 25

Solution

It is easy to place 20122012 points for the condition of the problem being fulfilled: we will place 20112011 points on a straight line, and the last one — outside of it (fig. 24).

Proof. Let's draw all the lines through every pair of points from the set MM. We will find a line and a point from MM such that the distance between them is minimal (excluding any pair where the point lies on the line). Let's denote the corresponding line as ll and the point as AA. We will prove by contradiction that there are only two points from MM lying on line ll.
Let's assume that there are at least three points lying on ll from the set MM, and let's denote them as "from left to right" as B1B_1, B2B_2 and B3B_3 (fig. 25). Since AB1+AB3>B1B3=B1B2+B2B3AB_1 + AB_3 > B_1B_3 = B_1B_2 + B_2B_3, then either AB1>B1B2AB_1 > B_1B_2 or AB3>B2B3AB_3 > B_2B_3. Without loss of generality, we assume that AB3>B2B3AB_3 > B_2B_3. Let us denote by h1h_1 – the height of the triangle AB2B3AB_2B_3 corresponding to the vertex AA, h2h_2 – the height of the same triangle corresponding to the vertex B2B_2. Then the area of the triangle AB2B3AB_2B_3 is given by S=12h1B2B3=12h2AB3S = \frac{1}{2}h_1 \cdot B_2B_3 = \frac{1}{2}h_2 \cdot AB_3, where h1>h2h_1 > h_2 when B2B3<AB3B_2B_3 < AB_3. However it implies that the distance between point B2B_2 and line AB3AB_3 is less than the distance between AA and ll. We obtain a contradiction which finishes the proof of the lemma.

Now we will prove that having nn points on the plane implies that either all of them belong to the same line or there exists at least nn different lines such that each contains at least two points among nn given. This will lead to the answer 20122012 for the question asked in this problem.
We will prove this statement by mathematical induction. If n=3n=3 then it is obvious. Suppose that the statement of induction is true for some value nn. Then we will prove the step of induction for n+1n+1. Assume that we have arbitrary n+1n+1 points on the plane, not all of which lie on the same line. According to the lemma we can choose two points such that the line through them does not contain any other points. Let us delete one of those two points such that all other points are not on the same line. After this operation we have nn points that satisfy the conditions of mathematical induction. Therefore for a new set of points we can find at least nn different lines such that each contains at least 22 of given points. Adding the line constructed in the beginning we will get at least n+1n+1 different lines for the initial set of n+1n+1 points, which finishes the proof of the step of induction.

Answer: 20122012 points.

Figure 1
Fig. 24

Figure 2
Fig. 25

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