Thursday, December 21, 2017

calculus - Indeterminate form 00 using L'hospitals rule when calculating limxto0+xsin(x)


Given the question lim



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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