Summation of triangle sides!

Algebra Level 5

Let T T be the set of all triples ( a , b , c ) (a,b, c) of positive integers such that there exist triangles with side lengths a , b , c a,b,c . If

( a , b , c ) T 2 a 3 b 5 c = α β \large \sum_{(a,b,c)\in T} \frac{2^a}{3^b 5^c} = \frac \alpha \beta

where α \alpha ane β \beta are positive coprime integers. What is α + β \alpha+\beta ?


The answer is 38.

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1 solution

Mark Hennings
Jul 6, 2018

This comes from summing a large number of GP series. Firstly, the triangle could be equilateral. Then S 1 = a = 1 2 a 3 a 5 a = 2 13 S_1 \; =\; \sum_{a=1}^\infty \frac{2^a}{3^a5^a} \; = \; \tfrac{2}{13} Next consider the cases where the triangle is isosceles, we have S 2 , 1 = ( a , b , b ) T , a < b 2 a 3 b 5 b = a = 1 b = a + 1 2 a 1 5 b = 1 91 S 2 , 2 = ( b , a , b ) T , a < b 2 b 3 a 5 b = 4 39 S 2 , 3 = ( b , b , a ) T , a < b 2 b 3 b 5 a = 4 13 S 2 , 4 = ( a , b , b ) T , a > b 2 a 3 b 5 b = b = 2 a = b + 1 2 b 1 2 a 1 5 b = 8 143 S 2 , 5 = ( b , a , b ) T , a > b 2 b 3 a 5 b = 4 559 S 2 , 6 = ( b , b , a ) T , a > b 2 b 3 b 5 a = 4 949 \begin{aligned} S_{2,1} & = \; \sum_{(a,b,b) \in T \;,\; a < b}\frac{2^a}{3^b5^b} \; = \; \sum_{a=1}^\infty \sum_{b=a+1}^\infty \frac{2^a}{15^b} \; = \; \tfrac{1}{91} \\ S_{2,2} & = \; \sum_{(b,a,b)\in T\;,\; a < b} \frac{2^b}{3^a5^b} \; = \; \tfrac{4}{39} \\ S_{2,3} & = \; \sum_{(b,b,a)\in T\;,\; a < b} \frac{2^b}{3^b5^a} \; = \; \tfrac{4}{13} \\ S_{2,4} & = \; \sum_{(a,b,b) \in T \;,\; a > b}\frac{2^a}{3^b5^b} \; = \; \sum_{b=2}^\infty \sum_{a=b+1}^{2b-1} \frac{2^a}{15^b} \; = \; \tfrac{8}{143} \\ S_{2,5} & = \; \sum_{(b,a,b)\in T\;,\; a > b} \frac{2^b}{3^a5^b} \; = \; \tfrac{4}{559} \\ S_{2,6} & = \; \sum_{(b,b,a)\in T\;,\; a > b} \frac{2^b}{3^b5^a} \; = \; \tfrac{4}{949} \end{aligned} There are also six cases to consider when the triangle is scalene, with S 3 , 1 = ( a , b , c ) T , a < b < c 2 a 3 b 5 c = a = 2 b = a + 1 c = b + 1 a + b 1 2 a 3 b 5 c = 2 6643 S 3 , 2 = ( a , b , c ) T , a < c < b 2 a 3 b 5 c = 2 3913 S 3 , 3 = ( a , b , c ) T , b < a < c 2 a 3 b 5 c = 8 2847 S 3 , 4 = ( a , b , c ) T , b < c < a 2 a 3 b 5 c = 16 429 S 3 , 5 = ( a , b , c ) T , c < a < b 2 a 3 b 5 c = 8 559 S 3 , 6 = ( a , b , c ) T , c < b < a 2 a 3 b 5 c = 16 143 \begin{aligned} S_{3,1} & = \; \sum_{(a,b,c) \in T\;,\; a < b < c} \frac{2^a}{3^b5^c} \; = \; \sum_{a=2}^\infty \sum_{b=a+1}^\infty \sum_{c=b+1}^{a+b-1} \frac{2^a}{3^b5^c} \; = \; \tfrac{2}{6643} \\ S_{3,2} & = \; \sum_{(a,b,c) \in T\;,\; a < c < b} \frac{2^a}{3^b5^c} \; = \; \tfrac{2}{3913} \\ S_{3,3} & = \; \sum_{(a,b,c) \in T\;,\; b < a < c} \frac{2^a}{3^b5^c} \; = \; \tfrac{8}{2847} \\ S_{3,4} & = \; \sum_{(a,b,c) \in T\;,\; b < c < a} \frac{2^a}{3^b5^c} \; = \; \tfrac{16}{429} \\ S_{3,5} & = \; \sum_{(a,b,c) \in T\;,\; c < a < b} \frac{2^a}{3^b5^c} \; = \; \tfrac{8}{559} \\ S_{3,6} & = \; \sum_{(a,b,c) \in T\;,\; c < b < a} \frac{2^a}{3^b5^c} \; = \; \tfrac{16}{143} \end{aligned} Thus the desired sum is S 1 + j = 1 6 S 2 , j + j = 1 6 S 3 , j = 17 21 S_1 + \sum_{j=1}^6 S_{2,j} + \sum_{j=1}^6 S_{3,j} \; = \; \tfrac{17}{21} making the answer 17 + 21 = 38 17+21 = \boxed{38} .

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