find sum of multiples of a number
// C# program to find sum of multiples
// of a number up to N efficiently
using System;
class GFG {
// Function for calculating sum
// of multiples of a upto N
static int calculate_sum(int a, int N)
{
// Number of multiples
int m = N / a;
// sum of first m natural numbers
int sum = m * (m + 1) / 2;
// sum of multiples
int ans = a * sum;
return ans;
}
// Driver code
public static void Main()
{
int a = 7, N = 49;
Console.WriteLine("Sum of multiples of " + a +
" up to " + N + " = " + calculate_sum(a, N));
}
}