这类题都不太清楚如何做...

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

就是这类题都不是很清楚有什么比较快的方法去做...求指教...谢谢谢谢!!!!!

the sum of all even intergers between 99 and 301 is equal to 100 to 300, so

1) {(100+300)/2}  * {((300-100)/2)+1}= 200 * 101 = 20,200