Modern day Pythagorean Triples

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.

All integer side lengths; one of them is 2018. What is the sum of the radii of all the circles inscribed in such triangles? All integer side lengths; one of them is 2018. What is the sum of the radii of all the circles inscribed in such triangles?


The answer is 1398.

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1 solution

Maria Kozlowska
Feb 9, 2018

2018 = 2 × 1009 2018=2 \times 1009 where 1009 1009 is prime number. Primitive triple has odd hypotenuse and odd one leg. If 2018 2018 were to be part of a primitive triple it would have to be of form 2 m n 2 m n using Euclid formula for generating triples where m > n > 0 m>n>0 and only one of m , n 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.

1009 = 4 × 252 + 1 = m 2 + n 2 1009 = 4 \times 252 + 1=m^2 + n^2

Analysing all possibilities we get m = 28 , n = 15 m=28,n=15 . This gives inradius r = n ( m n ) = 15 × 13 = 195 r=n(m-n)=15 \times 13 = 195 . Now we consider triples with one leg equal to 1009 1009 . It would have to be in the form: 1009 = m 2 n 2 = ( m n ) ( m + n ) 1009=m^2-n^2=(m-n)(m+n) m n = 1 2 n + 1 = 1009 m = 505 , n = 504 r = 504 m-n=1 \Rightarrow 2 n + 1 = 1009 \Rightarrow m=505, n=504 \Rightarrow r=504 The answer is 2 ( 195 + 504 ) = 1398 2(195+504)=\boxed{1398}

Are you saying that the 2 possible values of the inradius are 195 and 504? Because I got 390 and 1008.

And I don't know why you need to double their sum (at the very end).

Pi Han Goh - 3 years, 4 months ago

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195 and 504 are for the primitive triples, so the actual values for 2018 triangle are 360 and 1008.

Maria Kozlowska - 3 years, 4 months ago

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Oh that's silly of me. Thanks for clearing things up.

Pi Han Goh - 3 years, 4 months ago

This is a brilliant solution!! I had to use Heron's formula, so where do you have this short formula for the inradius from?

Henry U - 2 years, 5 months ago

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