Euler problems/1 to 10

Problem 1

Add all the natural numbers below 1000 that are multiples of 3 or 5.

Solution:

problem_1 = sum [ x | x <- [1..1000], (x `mod` 3 == 0) ||  (x `mod` 5 == 0)]

Problem 2

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

Solution:

problem_2 = sum [ x | x <- fibs, x `mod` 2 == 0]
  where fibs = 1 : 1 : zipWith (+) fibs (tail fibs)

Problem 3

Find the largest prime factor of 317584931803.

Solution:

problem_3 = maximum [ x | x <- [1..round $ sqrt (fromInteger c)], c `mod` x == 0]
  where c = 317584931803

Problem 4

Find the largest palindrome made from the product of two 3-digit numbers.

Solution:

problem_4 = foldr max 0 [ x | y <- [100..999], z <- [100..999], let x = y * z, let s = show x, s == reverse s]

Problem 5

What is the smallest number divisible by each of the numbers 1 to 20?

Solution:

problem_5 = head [ x | x <- [2520,5040..], all (\y -> x `mod` y == 0) [1..20]]

Problem 6

What is the difference between the sum of the squares and the square of the sums?

Solution:

problem_6 = sum [ x^2 | x <- [1..100]] - (sum [1..100])^2

Problem 7

Find the 10001st prime.

Solution:

problem_7 = head $ drop 10000 primes
  where primes = 2:3:..

Problem 8

Discover the largest product of five consecutive digits in the 1000-digit number.

Solution:

problem_8 = undefined

Problem 9

Find the only Pythagorean triplet, {a, b, c}, for which a + b + c = 1000.

Solution:

problem_9 = head [a*b*c | a <- [1..500], b <- [a..500], c <- [1..(1000-a-b)], a + b + c == 1000,  a^2 + b^2 == c^2]

Problem 10

Calculate the sum of all the primes below one million.

Solution:

problem_10 = sum [ p | p <- primes, p < 1000000 ]