for any positive integer n, the sum of the first n positive integers equals [n(n+1)]/2.what is the sum of all the even integers between 99 and 301?

a. 10,100

b. 20,200

c. 22,650

d. 40,200

e. 45,150

答案是b, but why is that?

(100+300)*101/2

I used the formula learnt from high school, it's the easiest method, if you forgot, just check your book.