2015 AMC 10B Problems/Problem 24
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[hide]Problem
Aaron the ant walks on the coordinate plane according to the following rules. He starts at the origin facing to the east and walks one unit, arriving at . For , right after arriving at the point , if Aaron can turn left and walk one unit to an unvisited point , he does that. Otherwise, he walks one unit straight ahead to reach . Thus the sequence of points continues , and so on in a counterclockwise spiral pattern. What is ?
Solution 1
The first thing we would do is track Aaron's footsteps:
He starts by taking step East and step North, ending at after steps and about to head West.
Then he takes steps West and steps South, ending at ) after steps, and about to head East.
Then he takes steps East and steps North, ending at after steps, and about to head West.
Then he takes steps West and steps South, ending at after steps, and about to head East.
From this pattern, we can notice that for any integer he's at after steps, and about to head East. There are terms in the sum, with an average value of , so:
If we substitute into the equation: . So he has moves to go. This makes him end up at .
Solution 2
We are given that Aaron starts at , and we note that his net steps follow the pattern of in the -direction, in the -direction, in the -direction, in the -direction, in the -direction, in the -direction, and so on, where we add odd and subtract even.
We want , but it does not work out cleanly. Instead, we get that , which means that there are extra steps past adding in the -direction (and the final number we add in the -direction is ).
So .
We can group as .
Thus .
Solution 3
Looking at his steps, we see that he walks in a spiral shape. At the th step, he is on the bottom right corner of the square centered on the origin. On the th step, he is on the bottom right corner of the square centered at the origin. It seems that the is the bottom right corner of the square. This makes sense since, after , he has been on dots, including the point . Also, this is only for odd , because starting with the square, we can only add one extra set of dots to each side, so we cannot get even . Since , is the bottom right corner of the square. This point is over to the right, and therefore down, so . Since is ahead of , we go back spaces to .
Solution 4 (similar to 3)
We call the first point, the second, and so on. As in Solution 3, we see that he walks in a counterclockwise spiral. We see that his path is traced out by a series of squares with odd-length sides that contain each other. At step he is at ; at step he is at ; and so on. We see that at step where is an odd number, he is at where The closest to (we want which is the th step) is so we let On the th step, so he is at We see that he approached from the left, so backtrack steps to the left (i.e. subtracting from the x-coordinate). Thus, we have ~Technodoggo
Solution 5 (Similar to Solution 1)
The first thing is to see the amount of footsteps at points
We see that at he has taken 2 footsteps.
To get to , Aaron takes steps West, steps South, steps East, and steps North.
We can think about this in the following way. For Aaron to get from to , he takes steps = steps
We can find the total steps taken to get to a point
= ) works as a formula
We find that to get to this is 2+10+18...+ (4(21)+2)(21)= 1892. We then simply go steps to get to
and more to get to the 2015th step at or answer D
Solution 6
If we graph for a few values of , we begin to see a particular pattern for when is a perfect square. For example, , , , and so on. Similarly, , , , and so on. Using this, we can generate a pattern for even and odd squares:
We notice that is quite close to . By the same pattern for odd squares, . is just units to the left of . Therefore, .
See Also
2015 AMC 10B (Problems • Answer Key • Resources) | ||
Preceded by Problem 23 |
Followed by Problem 25 | |
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