Construct all distinct right triangles with integer side lengths such that one of the side lengths is 2018.
Find the sum of the radii of all the circles inscribed in those triangles.
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2 0 1 8 = 2 × 1 0 0 9 where 1 0 0 9 is prime number. Primitive triple has odd hypotenuse and odd one leg. If 2 0 1 8 were to be part of a primitive triple it would have to be of form 2 m n using Euclid formula for generating triples where m > n > 0 and only one of m , n values is odd. This is a contradiction. Therefore 2018 cannot be a part of any primitive triple. We will consider triples with 1009.
1 0 0 9 = 4 × 2 5 2 + 1 = m 2 + n 2
Analysing all possibilities we get m = 2 8 , n = 1 5 . This gives inradius r = n ( m − n ) = 1 5 × 1 3 = 1 9 5 . Now we consider triples with one leg equal to 1 0 0 9 . It would have to be in the form: 1 0 0 9 = m 2 − n 2 = ( m − n ) ( m + n ) m − n = 1 ⇒ 2 n + 1 = 1 0 0 9 ⇒ m = 5 0 5 , n = 5 0 4 ⇒ r = 5 0 4 The answer is 2 ( 1 9 5 + 5 0 4 ) = 1 3 9 8