2011 AMC 12B Problem 23

Attempt Problem 23 of the 2011 AMC 12B below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2011 AMC 12B solutions, or check the answer key.

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23.

A bug travels in the coordinate plane, moving only along the lines that are parallel to the xx-axis or yy-axis. Let A=(3,2)A=(-3, 2) and B=(3,2).B=(3, -2). Consider all possible paths of the bug from AA to BB of length at most 20.20. How many points with integer coordinates lie on at least one of these paths?

161161

185185

195195

227227

255255

Answer: C
Concepts:lattice pointcaseworksymmetry
Difficulty rating: 2390
Solution:

A lattice point X=(x,y)X=(x,y) lies on some path exactly when d=x3+x+3+y2+y+220. \begin{aligned} d&=|x-3|+|x+3| \\ &\quad {}+|y-2|+|y+2|\le20. \end{aligned} This expression is unchanged when xxx\to-x or yy,y\to-y, so we count points with x0,x\ge0, y0,y\ge0, multiply by 4,4, and correct for the axes.

If 0x30\le x\le3 and 0y2,0\le y\le2, all 43=124\cdot3=12 points work. If 0x30\le x\le3 and y3,y\ge3, then y7,y\le7, giving 45=204\cdot5=20 points. If x4x\ge4 and 0y2,0\le y\le2, then x8,x\le8, giving 53=155\cdot3=15 points. Finally, for x4x\ge4 and y3,y\ge3, the condition is x+y10,x+y\le10, giving 4+3+2+1=104+3+2+1=10 points. Thus there are 5757 in the first quadrant, including 1515 on the nonnegative axes. By symmetry the total is 4572153=195. 4\cdot57-2\cdot15-3=195.

Thus, the correct answer is C.

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