Sunday, 17 November 2013

real analysis - If $K = lim_{i to infty}frac{x_i}{y_i}$ and $lim_{i to infty} x_i = 0 $, then $lim_{i to infty} y_i =0$?




If $$K = \lim_{i \to \infty}\frac{x_i}{y_i}$$
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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