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jmc

algebra senior

Problem

A lattice point is a point whose coordinates are both integers. How many lattice points are on the boundary or inside the region bounded by and ?
Solution
The graph of the two equations is shown below:



We first find the values at which the two equations intersect. When , . Plugging this into the second equation to eliminate , we get . Factoring the left hand side gives , so (since we stated the was non-negative). By symmetry, the value of the left intersection is . So we just have to consider the integer values between these two bounds and find all integer values that make the point fall inside the region.

For , there is 1 point that works: . For , the value of is and the value of is , so all values between 1 and 5 inclusive work, for a total of 5 points. For , the value of is and the value of is , so all values between 0 and 6 inclusive work, for a total of 7 points. By symmetry, when , there are 5 points that work, and when , there is 1 point that works.

In total, there are lattice points in the region or on the boundary.
Final answer
19