Berkeley Math Circle Monthly Contest 6 #0hwm

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.

Counting two waysRecursion, bijection

Problem

Problem: Let $k$ be a positive integer. Prove that there exist POSITIVE integers $a_{0}, \ldots, a_{k}$ such that for all integers $x \geq 0$ ,

x^{k}=a_{0}\left(\begin{array}{l} x \\ k \end{array}\right)+a_{1}\left(\begin{array}{c} x+1 \\ k \end{array}\right)+\cdots+a_{k-1}\left(\begin{array}{c} x+k-1 \\ k \end{array}\right) \text{.}

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