Find the difference between the square of the sum and the sum of the squares of the first N natural numbers.
The square of the sum of the first ten natural numbers is (1 + 2 + ... + 10)² = 55² = 3025.
The sum of the squares of the first ten natural numbers is 1² + 2² + ... + 10² = 385.
Hence the difference between the square of the sum of the first ten natural numbers and the sum of the squares of the first ten natural numbers is 3025 - 385 = 2640.
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Problem 6 at Project Euler http://projecteuler.net/problem=6
open OUnit2 open Difference_of_squares let ae exp got _test_ctxt = assert_equal exp got let square_of_sum_tests = [ "square of sum 1" >:: ae 1 (square_of_sum 1); "square of sum 5" >:: ae 225 (square_of_sum 5); "square of sum 100" >:: ae 25502500 (square_of_sum 100); ] let sum_of_squares_tests = [ "sum of squares 1" >:: ae 1 (sum_of_squares 1); "sum of squares 5" >:: ae 55 (sum_of_squares 5); "sum of squares 100" >:: ae 338350 (sum_of_squares 100); ] let difference_of_squares_tests = [ "difference of squares 1" >:: ae 0 (difference_of_squares 1); "difference of squares 5" >:: ae 170 (difference_of_squares 5); "difference of squares 100" >:: ae 25164150 (difference_of_squares 100); ] let () = run_test_tt_main ( "difference of squares tests" >::: List.concat [square_of_sum_tests; sum_of_squares_tests; difference_of_squares_tests] )
let square_of_sum n = let sum = (1 + n) * n / 2 in sum * sum let rec sum_of_squares n = match n with | 0 -> 0 | _ -> n * n + sum_of_squares (n - 1) let difference_of_squares n = square_of_sum n - sum_of_squares n
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