what is the formula for finding the sum of even numbers?
The sum of even natural numbers between 1 and 2n can be written as the sum of 2*i from i=1 to i=n. Factoring out the 2, it is equal to 2 times the sum of i from i=1 to n. Using the formula for the sum of consecutive numbers, we get that the sum of all even numbers from 1 to 2n equals n*(n+1).