Sunday, 16 July 2017

How can I find limits without L'Hopital's Rule?




My question is, how can I evaluate limits without L'Hopital's Rule ?



$$\lim\limits_{x \to 0} \frac{\sin(\sqrt[3]{x})}{1-\cos x}$$


Answer



For small $x$, $1-\cos x= 2\sin^2\frac{x}{2}\approx\frac{x^2}{2}$ while $\sin\sqrt[3]{x}\approx\sqrt[3]{x}$, so $\frac{\sin\sqrt[3]{x}}{1-\cos x}\approx 2x^{-5/3}$ diverges as $x\to 0$. Thus the limit is $\infty$.


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