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
这道题解了半天都得不到B的结果，解析说
The sum of the even integers between 99 and 301 is the sum of the even integers from 100 through 300, or thesum of the 50th even integer through the 150th even integer. To get this sum, find the sum of the first 150 even integers and subtract the sum of the first 49 even integers.
怎么又转换到50到150的偶数求和了。。。
求问大神们，谢谢！
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发表于 2016-8-6 12:01:38
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