Round #545

What is the sum of integers from 1 to 100?

**Sum of Integers from 1 to 100** The sum of integers from 1 to 100 can be calculated using the formula for the sum of an arithmetic series: **Formula:** `sum = (n * (a1 + an)) / 2` Where: - `n` is the number of terms (100 in this case) - `a1` is the first term (1) - `an` is the last term (100) **Calculation:** ```csharp int n = 100; int a1 = 1; int an = 100; int sum = (n * (a1 + an)) / 2; Console.WriteLine(sum); ``` **Result:** The sum of integers from 1 to 100 is **5050**.

```csharp using System; class Program { static void Main() { int sum = SumOfIntegers(1, 100); Console.WriteLine($"The sum of integers from 1 to 100 is: {sum}"); } static int SumOfIntegers(int start, int end) { int sum = 0; for (int i = start; i <= end; i++) { sum += i; } return sum; } } ``` This C# program calculates the sum of integers from 1 to 100 using a simple `for` loop. The `SumOfIntegers` method takes two parameters, `start` and `end`, which define the range of integers to be summed. The `Main` method calls `SumOfIntegers` with the desired range and prints the result to the console. Alternatively, you can use the formula for the sum of an arithmetic series: `n * (a1 + an) / 2`, where `n` is the number of terms, `a1` is the first term, and `an` is the last term. Here's how you can implement it: ```csharp using System; class Program { static void Main() { int sum = SumOfIntegers(1, 100); Console.WriteLine($"The sum of integers from 1 to 100 is: {sum}"); } static int SumOfIntegers(int start, int end) { int n = end - start + 1; return n * (start + end) / 2; } } ``` This version of the program is more efficient, especially for large ranges, since it only requires a constant amount of calculations, regardless of the size of the range.