Let be the set of all triples of positive integers such that there exist triangles with side lengths . If
where ane are positive coprime integers. What is ?
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This comes from summing a large number of GP series. Firstly, the triangle could be equilateral. Then S 1 = a = 1 ∑ ∞ 3 a 5 a 2 a = 1 3 2 Next consider the cases where the triangle is isosceles, we have S 2 , 1 S 2 , 2 S 2 , 3 S 2 , 4 S 2 , 5 S 2 , 6 = ( a , b , b ) ∈ T , a < b ∑ 3 b 5 b 2 a = a = 1 ∑ ∞ b = a + 1 ∑ ∞ 1 5 b 2 a = 9 1 1 = ( b , a , b ) ∈ T , a < b ∑ 3 a 5 b 2 b = 3 9 4 = ( b , b , a ) ∈ T , a < b ∑ 3 b 5 a 2 b = 1 3 4 = ( a , b , b ) ∈ T , a > b ∑ 3 b 5 b 2 a = b = 2 ∑ ∞ a = b + 1 ∑ 2 b − 1 1 5 b 2 a = 1 4 3 8 = ( b , a , b ) ∈ T , a > b ∑ 3 a 5 b 2 b = 5 5 9 4 = ( b , b , a ) ∈ T , a > b ∑ 3 b 5 a 2 b = 9 4 9 4 There are also six cases to consider when the triangle is scalene, with S 3 , 1 S 3 , 2 S 3 , 3 S 3 , 4 S 3 , 5 S 3 , 6 = ( a , b , c ) ∈ T , a < b < c ∑ 3 b 5 c 2 a = a = 2 ∑ ∞ b = a + 1 ∑ ∞ c = b + 1 ∑ a + b − 1 3 b 5 c 2 a = 6 6 4 3 2 = ( a , b , c ) ∈ T , a < c < b ∑ 3 b 5 c 2 a = 3 9 1 3 2 = ( a , b , c ) ∈ T , b < a < c ∑ 3 b 5 c 2 a = 2 8 4 7 8 = ( a , b , c ) ∈ T , b < c < a ∑ 3 b 5 c 2 a = 4 2 9 1 6 = ( a , b , c ) ∈ T , c < a < b ∑ 3 b 5 c 2 a = 5 5 9 8 = ( a , b , c ) ∈ T , c < b < a ∑ 3 b 5 c 2 a = 1 4 3 1 6 Thus the desired sum is S 1 + j = 1 ∑ 6 S 2 , j + j = 1 ∑ 6 S 3 , j = 2 1 1 7 making the answer 1 7 + 2 1 = 3 8 .