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Printsmc
counting and probability senior
Problem
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?
(A)
(B)
(C)
(D)
Solution
We declare a point to make up for the extra steps that the bug has to move. If the point satisfies the property that , then it is in the desirable range because is the length of the shortest path from to and is the length of the shortest path from to . If , then satisfy the property. there are lattice points here. else let (and for because it is symmetrical) We set 8 as the upper bound for x because the shortest distance from to added to the shortest distance from to is . Since the minimum value for the difference between the y-coordinates is at , we get or . Thus, the upper and lower bounds for are and , respectively. Now we test each value for x satisfying and double the result because of symmetry. For , the possibles values of y are such that for a total of lattice points, for , the possibles values of y are such that for a total of lattice points, for , the possibles values of y are such that for a total of lattice points, for , the possibles values of y are such that for a total of lattice points, for , the possibles values of y are such that for a total of lattice points, Hence, there are a total of lattice points. One may also obtain the result by using Pick's Theorem(how?). (Suggestion)
Final answer
C