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Find all Pythagorean triangles with length or height less than or equal to 20

Pythagorean triangles are right angle triangles whose sides comply with the following equation:

a * a + b * b = c * c

where c represents the length of the hypotenuse, and a and b represent the lengths of the other two sides. Find all such triangles where a, b and c are non-zero integers with a and b less than or equal to 20. Sort your results by the size of the hypotenuse. The expected answer is:

[3, 4, 5]
[6, 8, 10]
[5, 12, 13]
[9, 12, 15]
[8, 15, 17]
[12, 16, 20]
[15, 20, 25]
haskell
import Data.List
import Control.Monad

pythTriangles :: [(Int,Int,Int)]
pythTriangles = do
a <- [1..20]
b <- [a+1..20]
c <- [1..2*b]
guard (a*a + b*b == c*c)
return (a,b,c)

cmpThird (_,_,a) (_,_,b)
| a < b = LT
| a == b = EQ
| otherwise = GT

main = mapM_ print (sortBy cmpThird pythTriangles)
import Data.Function
import Data.List

pythTriangles =
[(a,b,c) | a <- [1..20], b <- [a+1..20], c <- [1..2*b], a*a + b*b == c*c]

main = mapM_ print $ sortBy (compare `on` third) pythTriangles where
third (_,_,x) = x

Greatest Common Divisor

Find the largest positive integer that divides two given numbers without a remainder. For example, the GCD of 8 and 12 is 4.

haskell
8 `gcd` 12