How to solve the maths problem from Good Will Hunting

Full credit to Numberphile for a better explanation of the problem. I like my technique more, though.
The problem in Good Will Hunting - Nu...

Marty Gots a Plan by Kevin MacLeod is licensed under a Creative Commons Attribution license (https://creativecommons.org/licenses/...)
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Rhodesia by Twin Musicom is licensed under a Creative Commons Attribution license (https://creativecommons.org/licenses/...)
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