Is this $\lim \ln(f(x))=\ln(\lim(f(x))$ valid?

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Is this mathematically legit?

$$\lim_{x\to\infty}\ln(f(x))=\ln(\lim_{x\to\infty}(f(x))$$

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It depends. If $\lim_{x\to\infty}f(x)$ exists and is in the domain of $\ln$, then the equation is valid, because of a theorem about continuity and composition of a continuous function.

Otherwise, at least one side of the equation is not defined.