2011 AMC 12B Problem 23
Problem 23 of 25HarderGeometry
A bug travels in the coordinate plane, moving only along the lines that are parallel to the -axis or -axis. Let and Consider all possible paths of the bug from to of length at most How many points with integer coordinates lie on at least one of these paths?
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Solution
A lattice point lies on some path exactly when This expression is unchanged when or so we count points with multiply by and correct for the axes.
If and all points work. If and then giving points. If and then giving points. Finally, for and the condition is giving points. Thus there are in the first quadrant, including on the nonnegative axes. By symmetry the total is
Thus, the correct answer is C.