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I now that: $$\lim_{x\to 0}\frac{\sin x}{x}=1,$$ but I didnt now how to prove that:$$\lim_{x\to 0}\frac{1-\cos x}{x^2}$$

Please help me. Thanky very much for your help.

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    The are almost the same thing, since $1-\cos x = 2\sin^2\left(\frac{x}{2}\right)$. – 2017-01-02
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    If you know Taylor-series of $\cos x$ you can prove it easily. – 2017-01-02
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    Use e.g. $\frac{1-\cos x}{x^2}=\frac{1-\cos^2 x}{x^2(1+\cos x)}=(\frac{\sin x}{x})^2\frac{1}{1+\cos x}$ . – 2017-01-02
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    For $$\lim\limits_{x\to 0}\dfrac {\sin x}x=1$$This might help: http://math.stackexchange.com/questions/75130/how-to-prove-that-lim-limits-x-to0-frac-sin-xx-1 – 2017-01-02
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    L'Hopital?${}{}$ – 2017-01-02

4 Answers 4

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First method: $$\lim_{x\to 0}\frac{1-\cos x}{x^2}=\lim_{x\to 0}\left(\frac{1-\cos x}{x^2}\cdot\frac{1+\cos x}{1+\cos x}\right)=\lim_{x\to 0}\frac{1-\cos^2 x}{x^2(1+\cos x)}$$ $$=\lim_{x\to 0}\frac{\sin^2 x}{x^2(1+\cos x)}=\lim_{x\to 0}\frac{\sin x\cdot \sin x}{x\cdot x (1+\cos x)}=\lim_{x\to 0}\left(\frac{\sin x}{x}\cdot\frac{\sin x}{x}\cdot \frac{1}{1+\cos x}\right)$$ $$=\lim_{x\to 0}\frac{\sin x}{x}\cdot\lim_{x\to 0}\frac{\sin x}{x}\cdot\lim_{x\to 0}\frac{1}{1+\cos x}=1\cdot 1\cdot \frac{1}{1+\lim_{x\to 0} \cos x}=\frac{1}{2}$$

Second method: Use the trigonometrics formula, $1-\cos x=2\sin^2\frac{x}{2}$ we have:

$$\lim_{x\to 0}\frac{1-\cos x}{x^2}=\lim_{x\to 0}\frac{2\sin^2\frac{x}{2}}{x^2}$$ $$=2\lim_{x\to 0}\frac{\sin\frac{x}{2}\cdot \sin\frac{x}{2}}{\frac{x}{2}\cdot\frac{x}{2}\cdot 4}=\frac{2}{4}=\frac{1}{2}$$

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Hint:

Put $\cos x=1- 2\sin^2\frac{x}{2}$.

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    All too easy. (+1) – 2017-01-02
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Notice that this is a $0/0$ indeterminant form, so you can also apply L'Hopital's rule:

$\displaystyle \lim_{x \rightarrow 0 } \frac{1 - \cos(x)}{x^2} = \lim_{x \rightarrow 0 } \frac{ \sin(x)}{2x}$, and limit arithmetic tells us that this is equal to $\displaystyle \frac{1}{2} \left( \lim_{x \rightarrow 0 } \frac{\sin(x)}{x} \right)$.

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    This should be the first thing that occurs to anyone. +1. – 2017-01-02
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    mhm, then again people often prefer as little "machinery" as possible. – 2017-01-02
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A simpler hint for the second limit: $$\frac{1-\cos x}{x^2}=\frac{(1-\cos x)(1+\cos x)}{x^2(1+\cos x)}=\dotsm$$