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Geometry Difficulty 5.3 AIME, harder Find the answer

In the Cartesian plane, let A=(0,0),B=(200,100)A=(0,0), B=(200,100), and C=(30,330)C=(30,330). Compute the number of ordered pairs (x,y)(x, y) of integers so that (x+12,y+12)\left(x+\frac{1}{2}, y+\frac{1}{2}\right) is in the interior of triangle ABCA B C.

A number or a short expression. Spacing and $ signs are ignored.

Solution

We use Pick's Theorem, which states that in a lattice polygon with II lattice points in its interior and BB lattice points on its boundary, the area is I+B/21I+B / 2-1. Also, call a point center if it is of the form (x+12,y+12)\left(x+\frac{1}{2}, y+\frac{1}{2}\right) for integers xx and yy. The key observation is the following - suppose we draw in the center points, rotate 4545^{\circ} degrees about the origin and scale up by 2\sqrt{2}. Then, the area of the triangle goes to 2K2 K, 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 KK, let I1I_{1} and B1B_{1} be the number of interior lattice points and boundary lattice points, respectively. Let IcI_{c} and BcB_{c} be the number of interior and boundary points that are center points in the original triangle. Finally, let I2I_{2} and B2B_{2} 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, K=I1+B1/212K=I2+B2/21(I2I1)=KB1B22Ic=KBc2\begin{aligned} K & =I_{1}+B_{1} / 2-1 \\ 2 K & =I_{2}+B_{2} / 2-1 \\ \Longrightarrow\left(I_{2}-I_{1}\right) & =K-\frac{B_{1}-B_{2}}{2} \\ \Longrightarrow I_{c} & =K-\frac{B_{c}}{2} \end{aligned} One can compute that the area is 31500. The number of center points that lie on on AB,BCA B, B C, and CAC A are 0,10, and 30, respectively. Thus, the final answer is 315000+10+302=3148031500-\frac{0+10+30}{2}=31480.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.