In the Cartesian plane, let , and . Compute the number of ordered pairs of integers so that is in the interior of triangle .
Solution
We use Pick's Theorem, which states that in a lattice polygon with lattice points in its interior and lattice points on its boundary, the area is . Also, call a point center if it is of the form for integers and . The key observation is the following - suppose we draw in the center points, rotate degrees about the origin and scale up by . Then, the area of the triangle goes to , and the set of old lattice points and center points becomes a lattice. Hence, we can also apply Pick's theorem to this new lattice. Let the area of the original triangle be , let and be the number of interior lattice points and boundary lattice points, respectively. Let and be the number of interior and boundary points that are center points in the original triangle. Finally, let and be the number of interior and boundary points that are either lattice points or center points in the new triangle. By Pick's Theorem on both lattices, One can compute that the area is 31500. The number of center points that lie on on , and are 0,10, and 30, respectively. Thus, the final answer is .