一道Gmat数学题!

题目

一道Gmat数学题!
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?

答案

1~99的和为99*（99+1）/2=4950
1~301的和为301*（301+1）/2=45451
100~301的和为45451-4950=40501
100~301 设偶数和为x 则奇数和为x+101 （共202个数,奇数和偶数各101个 奇数和比偶数和大101）
x+(x+101)=40501
x=20200
即偶数和为20200
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