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• 12 Sum of alternating series is non-zero?
• 12 An old and interesting problem in combinatorics from Russia Mathematics Olympiad
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• 5 Let $A$ be a $n \times n$ matrix. Show that if $A^k=0$ then $\operatorname{rank} A \leq n(k-1)/k$
• 5 Incorrect combinatorics reasoning
• 5 If a diameter has endpoints (a,b) and (c,d), prove that the equation of the circle can be written $(x-a)(x-c)+(y-b)(y-d)=0$
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