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jmc

counting and probability intermediate

Problem

Consider the rectangular region with the following points as vertices: How many points with integer coordinates will be strictly in the interior of this rectangular region?
Solution
The rectangular region is 10 units by 8 units, resulting in an 8 by 6 rectangular region in the interior, which forms a 9-by-7 array of lattice points. That's points with integer coordinates, as shown in the figure. Note: We are counting points, not squares. It is a common mistake to count the interior squares, for 48, instead of interior lattice points, which gives the correct answer of 63.
Final answer
63