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Geometry Difficulty 5.0 AIME Find the answer

Compute the number of ways a non-self-intersecting concave quadrilateral can be drawn in the plane such that two of its vertices are (0,0)(0,0) and (1,0)(1,0), and the other two vertices are two distinct lattice points (a,b),(c,d)(a, b),(c, d) with 0a,c590 \leq a, c \leq 59 and 1b,d51 \leq b, d \leq 5.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

We instead choose points (0,0),(1,0),(a,b),(c,d)(0,0),(1,0),(a, b),(c, d) with 0a,c590 \leq a, c \leq 59 and 0b,d50 \leq b, d \leq 5 with (c,d)(c, d) in the interior of the triangle formed by the other three points. Any selection of these four points may be connected to form a concave quadrilateral in precisely three ways. Apply Pick's theorem to this triangle. If II is the count of interior points, and BB is the number of boundary lattice points, we have that the triangle's area is equal to b2=I+B21\frac{b}{2}=I+\frac{B}{2}-1 Let's first compute the number of boundary lattice points on the segment from (0,0)(0,0) to (a,b)(a, b), not counting (0,0)(0,0). This is just gcd(a,b)\operatorname{gcd}(a, b). Similarly, there are gcd(a1,b)\operatorname{gcd}(a-1, b) boundary lattice points from (1,0)(1,0) to (a,b)(a, b). Adjusting for the overcounting at (a,b)(a, b), we have B=gcd(a,b)+gcd(a1,b)1B=\operatorname{gcd}(a, b)+\operatorname{gcd}(a-1, b)-1 and thus I=bgcd(a,b)gcd(a1,b)+12I=\frac{b-\operatorname{gcd}(a, b)-\operatorname{gcd}(a-1, b)+1}{2} which we notice is periodic in aa with period bb. That is, the count of boundary points does not change between choices (a,b)(a, b) and (a+b,b)(a+b, b). We wanted to find the sum across all (a,b)(a, b) of II, the number of interior points (c,d)(c, d). Using casework on bb, the periodicity allows us to just check II across points with 0a<b0 \leq a<b, and then multiply the count by 60b\frac{60}{b} to get the sum of II across the entire row of points. For b=1,2b=1,2, we always have I=0I=0. For b=3b=3, we have I=0I=0 at (0,3),(1,3)(0,3),(1,3) and I=1I=1 for (2,3)(2,3). Using periodicity, this yy-coordinate has a total a total of (0+0+1)603=20(0+0+1) \cdot \frac{60}{3}=20 For b=4b=4, we have I=0I=0 at (0,4)(0,4) and (1,4)(1,4), and I=1I=1 at both (2,4)(2,4) and (3,4)(3,4). Using periodicity, this yy-coordinate has a total of (0+0+1+1)604=30(0+0+1+1) \cdot \frac{60}{4}=30 For b=5b=5, we have I=0I=0 at (0,5),(1,5)(0,5),(1,5) and I=2I=2 at (2,5),(3,5),(4,5)(2,5),(3,5),(4,5). Using periodicity, this yy-coordinate has a total of (0+0+2+2+2)605=72(0+0+2+2+2) \cdot \frac{60}{5}=72 Adding our cases, we have 20+30+72=12220+30+72=122 ways to choose the four points. Multiplying back by the number of ways to connect the quadrilateral gives an answer of 1223=366122 \cdot 3=366.

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