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alphaNew13
Bunuel can you pls explain the solution
­
Bunuel
If \(S = \frac{2001}{2*3} + \frac{2001}{3*4} + \frac{2001}{4*5} + ... + \frac{2001}{19*20}\), what is the value of S ?

A. 700.45
B. 800.55
C. 900.45
D. 1000.75
E. 1100.45
­
The point is:

    \( \frac{1}{n }- \frac{1}{n + 1} =\) 

    \(= \frac{(n + 1) - n}{n(n + 1) } = \)

    \(= \frac{1}{n(n + 1)}\)­
­­

Hence, \(\frac{1}{2*3}\)­ can be written as \(\frac{1}{2} - \frac{1}{3}\)­­­. Similarly, \(\frac{1}{3*4}\) can be written as \(\frac{1}{3} - \frac{1}{4}\). And so on. 

Therefore: 

    \(\frac{2001}{2*3} + \frac{2001}{3*4} + \frac{2001}{4*5} + ... + \frac{2001}{19*20} = \)

    \(=2001(\frac{1}{2*3} + \frac{1}{3*4} + \frac{1}{4*5} + ... + \frac{1}{19*20}) = \)

    \(=2001((\frac{1}{2} - \frac{1}{3}) + (\frac{1}{3} - \frac{1}{4}) + (\frac{1}{4} - \frac{1}{5}) + ... + (\frac{1}{19} - \frac{1}{20}))\)­
 ­
Observe that the fractions starting from the second one will be canceled with the following fractions. Thus, we'd be left only with the first fraction, \(\frac{1}{2}\), and with the last fraction, \(- \frac{1}{20}\), because there is no next fraction for it to be canceled by. So, we'd be left:

    \(=2001(\frac{1}{2} - \frac{1}{20})=\)

    \(=2001(\frac{18}{40})=\)

    \(= 900.45\)
    ­

Answer: C.

Hope it helps.


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