Problem 2

Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:

1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...

Find the sum of all the even-valued terms in the sequence which do not exceed four million.

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problem2 =
  sum [n | n <- takeWhile (<=4000000) fibs, even n]
  where
  fibs = 1 : 1 : zipWith (+) fibs (tail fibs)

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