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Printsmc
counting and probability senior
Problem
Parallelogram has area . Vertex is at and all other vertices are in the first quadrant. Vertices and are lattice points on the lines and for some integer , respectively. How many such parallelograms are there? (A lattice point is any point whose coordinates are both integers.)
(A)
(B)
(C)
(D)
Solution
The area of any parallelogram can be computed as the size of the vector product of and . In our setting where , , and this is simply . In other words, we need to count the triples of integers where , and . These can be counted as follows: We have identical red balls (representing powers of ), blue balls (representing powers of ), and three labeled urns (representing the factors , , and ). The red balls can be distributed in ways, and for each of these ways, the blue balls can then also be distributed in ways. (See Distinguishability for a more detailed explanation.) Thus there are exactly ways how to break into three positive integer factors, and for each of them we get a single parallelogram. Hence the number of valid parallelograms is .
Final answer
C