# Finding the sum of a series $\frac{1}{1 \cdot 2}-\frac{1}{2 \cdot 3}+\frac{1}{3 \cdot 4}-\frac{1}{4 \cdot 5}+…$

The value of:

$$\frac{1}{1 \cdot 2} - \frac{1}{2 \cdot 3} + \frac{1}{3 \cdot 4} - \frac{1}{4 \cdot 5} + \cdots$$

is

(A) between 0 and 1/4.

(B) between 1/4 and 1/3.

(C) between 1/3 and 1/2.

(D) between 1/2 and 1.


I was looking for a convenient way to tackle it. Can someone point me in the right direction? Thanks in advance for your time.

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Which problem would you like someone to solve? – Ron Gordon Jan 21 '13 at 14:06
sorry for the wrong post.I have edited it. – learner Jan 21 '13 at 14:09
Isn't it time for you to have learned TeX, rather than posting images? – Thomas Andrews Jan 21 '13 at 14:09
Does the above really mean $1.2=\frac{12}{10}$ or does it mean $1\times 2$? – Thomas Andrews Jan 21 '13 at 14:13
It means $1\times 2$ – learner Jan 21 '13 at 14:19

This is an alternating series with monotonely (ignoring the signs) decreasing terms. The proof of Leibniz' test shows that the value of the series $s$ lies between two consecutive partial sums.
Hence $$\frac{1}{2} > s > \frac{1}{2}-\frac{1}{6} = \frac{1}{3}.$$
It's same as $(1-\frac{1}{2})-(\frac{1}{2}-\frac{1}{3})+(\frac{1}{3}-\frac{1}{4})-(\frac{1}{4}-\frac{1}{5})+\cdots=1+2(-\frac{1}{2}+\frac{1}{3}-\frac{1}{4}+\frac{1}{5}-\cdots)$
$=1+2(\ln2-1)=2\ln2-1\approx 0.3862$