AMC 10 Step by Step
2015 AMC 10BProblem 24P21-25~7 minPrint

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=1,2,3,n=1,2,3,\dots , right after arriving at the point pnp_n , if Aaron can turn 9090^\circ left and walk one unit to an unvisited point pn+1p_{n+1} , he does that. Otherwise, he walks one unit straight ahead to reach pn+1p_{n+1} . Thus the sequence of points continues p2=(1,1),p3=(0,1),p4=(1,1),p5=(1,0)p_2=(1,1), p_3=(0,1), p_4=(-1,1), p_5=(-1,0) , and so on in a counterclockwise spiral pattern. What is p2015p_{2015} ?

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Problem © Mathematical Association of America (MAA), American Mathematics Competitions. Reproduced for non-commercial educational use. Solution and commentary are original to this site.