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2011 AMC 12B Problem 23

Problem 23 of 25HarderGeometryProblem-Solving Techniques

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?

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Solution

A lattice point X=(x,y)X=(x,y) lies on some path exactly when d=∣x−3∣+∣x+3∣+∣y−2∣+∣y+2∣≤20. \begin{aligned} d&=|x-3|+|x+3| \\ &\quad {}+|y-2|+|y+2|\le20. \end{aligned} This expression is unchanged when x→−xx\to-x or y→−y,y\to-y, so we count points with x≥0,x\ge0, y≥0,y\ge0, multiply by 4,4, and correct for the axes. If 0≤x≤30\le x\le3 and 0≤y≤2,0\le y\le2, all 4⋅3=124\cdot3=12 points work. If 0≤x≤30\le x\le3 and y≥3,y\ge3, then y≤7,y\le7, giving 4⋅5=204\cdot5=20 points. If x≥4x\ge4 and 0≤y≤2,0\le y\le2, then x≤8,x\le8, giving 5⋅3=155\cdot3=15 points. Finally, for x≥4x\ge4 and y≥3,y\ge3, the condition is x+y≤10,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 4⋅57−2⋅15−3=195. 4\cdot57-2\cdot15-3=195. Thus, the correct answer is C.
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Tagged: lattice point · casework · symmetry

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