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.