How to find the sum of odd numbers?

Finding the sum of odd numbers is straightforward and can be done using a formula. Let's explore the methods:

Formula Method

According to the reference, there are two ways to calculate the sum of odd numbers using a formula:

Sn= n/2 × [a + l]
- Where:
  - Sn is the sum of the odd numbers.
  - n is the number of odd numbers.
  - a is the first odd number in the sequence.
  - l is the last odd number in the sequence.
Sn = n²
- Where:
  - Sn is the sum of the odd numbers.
  - n is the number of odd numbers.

To calculate the sum of odd numbers between 1 and 20, we know there are 10 odd numbers (1, 3, 5, 7, 9, 11, 13, 15, 17, 19). Therefore, n = 10.

Sn = 10² = 100.

So, the sum of odd numbers between 1 and 20 is 100.

Method	Formula	Variables	Example (1 to 20)	Result
Formula 1	Sn= n/2 × [a + l]	n=10, a=1, l=19	10/2 * (1 + 19)	100
Formula 2	Sn= n²	n=10	10²	100