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Even Fibonacci Numbers

First read the problem description.
  1. Every third fibonacci number is even, so we do a simple loop over all the fibonacci numbers smaller than \(4000000\) and sum every third one.

  2. Every third fibonacci number is even. Since \(F_n = F_{n-3} * 4 + F_{n-6}\), we initialy calculate \(F_3\) and \(F_6\) then \(F_9 = F_6 * 4 + F_3\), etc…


Source code of the solution(s):