Pick's Theorem
From AoPSWiki
Pick's Theorem expresses the area of a polygon, all of whose vertices are lattice points in a coordinate plane, in terms of the number of lattice points inside the polygon and the number of lattice points on the sides of the polygon. The formula is:
where 
is the number of lattice points in the interior and 
being the number of lattice points on the boundary.
It is similar to the shoestring formula, and though it is less powerful it is a good tool to have in solving problems.
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Proof
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