In the sequence of positive numbers \(x_1\), \(x_2\), \(x_3\), ..., what is the value of \(x_1\)?(1) \(x_i=\frac{x_{(i-1)}}{2}\) for all integers \(i>1\) --> we have the general formula connecting two consecutive terms (
basically we have geometric progression with common ratio 1/2), but without the value of any term this info is insufficient to find \(x_1\).
(2) \(x_5=\frac{x_4}{x_4+1}\) --> we have the relationship between \(x_5\) and \(x_4\), also insufficient to find \(x_1\) (we cannot extrapolate the relationship between \(x_5\) and \(x_4\) to all consecutive terms in the sequence).
(1)+(2) From (2):
From (1):
Thus:
\(\frac{x_4}{2}=\frac{x_4}{x_4+1}\);
\(\frac{1}{2}=\frac{1}{x_4+1}\);
\(x_4=1\).
Since from (1) \(x_i=\frac{x_{(i-1)}}{2}\), then:
\(x_4=\frac{x_3}{2}=\frac{x_2}{4}=\frac{x_1}{8}\).
So:
\(x_4=\frac{x_1}{8}\);
\(1=\frac{x_1}{8}\);
\(x_1=8\).
Sufficient.
Answer: C.