Sunday, 17 November 2013

real analysis - If K=limitoinftyfracxiyi and limitoinftyxi=0, then limitoinftyyi=0?




If K=lim
with $0and \lim_{i \to \infty} x_i = 0
then will it be the case that
\lim_{i \to \infty} y_i =0
? I tried to prove this using different properties of limit, but so far I found no way. L'hopital's rule applies only when we can assume limits of both denominator and numerator exists or are infinite...



Edit: the main problem I observe in proving and disproving is eliminating/confirming possibility where the limit of y_i as i goes to infinity does not exist.


Answer




Since \lim_{i \to \infty}\frac{x_i}{y_i} = K and 0 < K < \infty we have that \lim_{i \to \infty}\frac{y_i}{x_i} = \frac{1}{K} = L, where 0 < L < \infty



Then we have:



\lim_{i \to \infty} y_i = \lim_{i \to \infty} \frac{y_i}{x_i} \cdot \lim_{i \to \infty} x_i = L \cdot 0 = 0


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