大家看看这个jj怎么做~谢谢

predy1

一个数列, a_n=
            n-1/(n+1), 求前50项的和为多少

suman_wu

应该按照这样：an=n-(1/(n+1))来理解吧，怎么做呢

gyyygyyy

好难啊 做不出来

gyyygyyy

以下是引用suman_wu在2008-9-8 9:55:00的发言：
应该按照这样：an=n-(1/(n+1))来理解吧，怎么做呢

n-(1/(n+1))  和 (n-1)/(n+1) 我都试了 做不出来 能力有限

dianecarton

a1=? n的取值范围大于几？

这个题目不完整

wxy831

Prepare GMAT' math does not require the knowledge of (1+1/2+1/3+1/4...+1/50)

So how to handle this one?

I will do like this

first we can deduce this one easily 50-2(1/2+1/3+1/4+1/5+1/6....+1/51)

now remeber it's impossible for us to work out a accurate answer, just follow these steps

1/2 = 0.5   1/3=0.33  1/4=0.25  1/5=0.125  1/6=0.16  1/7=0.14  1/8=0.125 ... 1/10=0.1

all of the first ten numbers sum up about less than 2

let's go on!  1/10+1/12+.....+1/19<10*0.1=1    1/20+1/22+.....+1/29<10*0.05=0.5  1/30+...+1/39<10*0.033=0.3 1/40+...+1/49<0.25  so,let's sum up  .... en about 2

so we work out about 4  the final answer is 50-2*4 = 42 .  So we can choose the answer near 42, I believe the five choices will like A 10 B 25 C 43 D 48 E 50 

actually 1/2+1/3+...+1/51=ln51+0.57721-1=3.50903

the accurate answer is 50-2*3.5=43

But on one can calculate like that, all we can do just work out a approximate answer. The ability of estimation is very important for this examination when we encounter a question like this.

Santavie

D楼上的~

jesuisdesole

有人能帮我这个数学白痴解释下怎么从给出的式子推出50-2(1/2+1/3+1/4+1/5+1/6....+1/51)这一步的么?

感激不尽