涛涛GWD31套讨论贴

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

此题有什么简单的做法啊，大家帮忙看看啊。

a1=100

an=98+2n

n=(300-100)/2 +1=101

sn=(a1+an)n/2

  =(100+98+2*101)*101/2

  =20200

(100+300)*101/2

前面的条件相当于告诉你这是等差数列呗，首相加末相乘以项数除以二 ,一目了然。

哦，好的，看来数列这块还得补补