Difference between revisions of "2022 SSMO Speed Round Problems/Problem 7"

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==Problem==
  
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Let <math>A_1=(1, 0)</math>. Define <math>A_{n+1}</math> as the image of <math>A_n</math> under a rotation of either <math>45^{\circ}</math>, <math>90^{\circ}</math>, or <math>135^{\circ}</math> clockwise about the origin, with each choice having a <math>\frac{1}{3}</math> chance of being selected. Find the expected value of the smallest positive integer <math>n>1</math> such that <math>A_n</math> coincides with <math>A_1</math>.
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==Solution==

Latest revision as of 19:14, 2 May 2025

Problem

Let $A_1=(1, 0)$. Define $A_{n+1}$ as the image of $A_n$ under a rotation of either $45^{\circ}$, $90^{\circ}$, or $135^{\circ}$ clockwise about the origin, with each choice having a $\frac{1}{3}$ chance of being selected. Find the expected value of the smallest positive integer $n>1$ such that $A_n$ coincides with $A_1$.

Solution