Thursday, December 21, 2017

calculus - Indeterminate form $0^0$ using L'hospitals rule when calculating $lim_{xto0^+} x^{sin(x)}$


Given the question $$\lim_{x\to0^+} x^{\sin(x)}$$



I have deducted so far that this has the indeterminate form $0^0$ so I have taken the natural logarithm of both sides to give me: $$\lim_{x\to0^+} \ln(y) = \lim_{x\to0^+}\sin(x)\ln(x)$$ This still has an indeterminate form so I rearrange it to start using L'hospitals rule: $$\lim_{x\to0^+}\frac{\ln(x)}{\csc(x)}$$ $$=\lim_{x\to0^+}\frac{\frac1x}{-\csc(x)\cot(x)}$$ $$=\lim_{x\to0^+}\frac{-\sin(x)\tan(x)}{x}$$ $$=\lim_{x\to0^+}\frac{-\cos(x)\sec^2(x)}{1} = -1$$ $$\lim_{x\to0^+}x^{\sin(x)} = \frac1e$$


I don't think this is correct, am I doing something wrong, and if so, where?


Answer




$$\lim_{x\to0^+} x^{\sin(x)}$$



$$=\lim_{x\to0^+} \exp(\ln(x)\sin(x))$$


$$=\exp(\lim_{x\to0^+} \ln(x)\sin(x))$$


$$=\exp(\lim_{x\to0^+} \frac{\ln(x)}{1/\sin(x)})$$


Applying L'Hôpital's rule:



$$=\exp(\lim_{x\to0^+} -\frac{\sin(x)^2}{x\cos(x)})$$


$$=\exp(-1\cdot \lim_{x\to0^+} \frac{1}{\cos(x)}\lim_{x\to0^+} \frac{\sin(x)^2}{x})$$


$$=\exp(-\lim_{x\to0^+} \frac{\sin(x)^2}{x})$$


Applying L'Hôpital's rule:


$$=\exp(-\lim_{x\to0^+} 2\cos(x)\sin(x))$$


$$\color{grey}{e^0=1}$$


$$=\boxed {\color{blue}1}$$


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