Carcass wrote:
If \(F(X) = \frac{1}{X} - \frac{1}{X+1}\), what is the sum of F(1) through F(200), inclusive?
A. 0
B. 1
C. 100
D. 200/201
E. 1/40200
Kudos for the right answer and explanation
Question part of the project GRE Quantitative Reasoning Daily Challenge - (2021) EDITIONGRE - Math BookFrom the given information, we get...
\(f(1) = 1 - \frac{1}{2}\)
\(f(2) = \frac{1}{2} - \frac{1}{3}\)
\(f(3) = \frac{1}{3} - \frac{1}{4}\)
\(f(4) = \frac{1}{4} - \frac{1}{5}\)
.
.
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\(f(199) = \frac{1}{199} - \frac{1}{200}\)
\(f(200) = \frac{1}{200} - \frac{1}{201}\)
So, \(f(1) + f(2) + f(3) + f(4) + . . . . f(199) + f(200) = (1 - \frac{1}{2}) + (\frac{1}{2} - \frac{1}{3}) + (\frac{1}{3} - \frac{1}{4}) + (\frac{1}{4} - \frac{1}{5}) . . . . . + (\frac{1}{199} - \frac{1}{200}) + (\frac{1}{200} - \frac{1}{201}) \)
Notice that almost every POSITIVE fraction has an identical NEGATIVE fraction.
So all of those fractions add up to be ZERO
The only two values that don't cancel out are the first and last terms (1 and -1/201)
In other words, the sum \(= 1 - \frac{1}{201} = \frac{201}{201}-\frac{1}{201}=\frac{200}{201}\)
Answer: D