Berkeley Math Circle Monthly Contest 6 #0hug

Berkeley Math Circle Monthly Contest 6Hard

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Understand the scale

Scale 0.5: Easiest math competition problems, often solvable even for early middle school students without experience. 1: Problems strictly for beginners, often at a moderate middle school level. 1.5: Problems for stronger beginner students, on the level of the middling problems in most middle school contests. 2: For motivated beginners, harder questions from the previous categories. 2.5: More advanced beginner problems, hardest questions from previous categories. 3: Early intermediate problems that require more creative thinking. 3.5: Tougher early intermediate problems that consistently stretch into higher-level creative thinking or conceptual knowledge. 4: Intermediate-level problems. 4.5: Upper intermediate problems approaching the upper bound for non-invitational sprint math competitions. 5: More difficult AIME problems or simple proof-based Olympiad-style problems. 6: High-level AIME-styled questions or introductory Olympiad-level questions. 7: Tougher Olympiad-level questions, may require more technical knowledge. 8: High-level Olympiad-level questions. 9: Expert Olympiad-level questions. 9.5: The hardest problems appearing on Olympiads which the strongest students could reasonably solve. 10: Historically notorious problems, generally unsuitable for even very hard competitions because they are exceedingly tedious, long, and difficult.

Problem

Problem: Let $A_{0}, A_{1}, \ldots, A_{n}$ be points in a plane such that (i) $A_{0}A_{1} \leq \frac{1}{2}A_{1}A_{2} \leq \cdots \leq \frac{1}{2^{n-1}}A_{n-1}A_{n}$ and (ii) $0 < \measuredangle A_{0}A_{1}A_{2} < \measuredangle A_{1}A_{2}A_{3} < \cdots < \measuredangle A_{n-2}A_{n-1}A_{n} < 180^{\circ}$ , where all these angles have the same orientation. Prove that the segments $A_{k}A_{k+1}, A_{m}A_{m+1}$ do not intersect for each $k$ and $n$ such that $0 \leq k \leq m-2 < n-2$ .