The sum of odd numbers formula is S_n= n/2 [a + l] where 'a' is the first odd number, 'l' is the last odd number and 'n' is the number of odd numbers present in that range. Another formula to calculate the sum of first n odd numbers is S_n= n².

Even number is divisible by 2, and leaves the remainder 0. An odd number is not completely divisible by 2, and leaves the remainder 1. An even number ends with 0, 2, 4, 6, and 8. An odd number ends with 1, 3, 5, 7, and 9.