# Project Euler – Problem 75 Solution

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The problem description is here, and click here to see all my other Euler solutions in F#.

I based my solution on Euclid’s formula for generating Pythagorean triples. And given that max L is 1,500,000, the maximum value for m we need to consider is $\sqrt{\frac{L}{2}}$. Because $L = a + b + c$ and $a^2 + b^2 = c^2$, we can deduce that $c < \frac{L}{2}$; and since $c = m^2 + n^2$ we also have $m < \sqrt{c}$ and therefore $m < \sqrt{\frac{L}{2}}$. The above makes use of a recursive function to calculate the GCD (based on Euclidean Algorithm): For efficiency, we’ll create a cache to store the number of ways L can be used to create integer sided right-angle triangle. As we iterate through the m and n pairs we generated above, we’ll take advantage of the fact that if $a^2 + b^2 = c^2$ then $ka^2 + kb^2 = kc^2$ must also be true and increment multiples of L by one. Finally, to work out the answer: This solution runs for about 350ms on my machine.

The source code for this solution is here. Hi, I’m Yan. I’m an AWS Serverless Hero and the author of Production-Ready Serverless.

I specialise in rapidly transitioning teams to serverless and building production-ready services on AWS.

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