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2015 AMC 10B Problem 24

Problem 24 of 25HarderGeometryNumber Theory

Aaron the ant walks on the coordinate plane according to the following rules. He starts at the origin p0=(0,0)p_0=(0,0) facing to the east and walks one unit, arriving at p1=(1,0).p_1=(1,0). For n=1,n=1, 2,2, 3,3, ,\dots, right after arriving at the point pn,p_n, if Aaron can turn 9090^\circ left and walk one unit to an unvisited point pn+1,p_{n+1}, he does that. Otherwise, he walks one unit straight ahead to reach pn+1.p_{n+1}. Thus the sequence of points continues p2=(1,1),  p3=(0,1),p4=(1,1),  p5=(1,0),   \begin{aligned} &p_2=(1,1),\; p_3=(0,1), \\ &p_4=(-1,1),\; p_5=(-1,0),\;\ldots \end{aligned} in a counterclockwise spiral pattern. What is p2015?p_{2015}?

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Solution

When Aaron reaches (k,k)(k,-k), he has just completed the square spiral containing all grid points with coordinates between k-k and kk. Therefore p(2k+1)21=(k,k).p_{(2k+1)^2-1}=(k,-k). With k=22k=22, this gives p2024=(22,22)p_{2024}=(22,-22). Since 20242015=92024-2015=9, stepping backward along the bottom edge subtracts 99 from the xx-coordinate, so p2015=(13,22).p_{2015}=(13,-22). Thus, the correct answer is D.

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Concepts: coordinate geometry · pattern recognition · perfect square

Problem text and solution from the LIVE past-contest archive. See also the AoPS wiki page for community solutions.