# Thread: limit as x goes to zero of sin(x^2)/x

1. ## limit as x goes to zero of sin(x^2)/x

Does this require l'Hospitals's Rule or is there another way to do this?

l'hospital's rule would give lim x->0 cos(x^2)/1 which is = 1/1=1

2. ## Re: limit as x goes to zero of sin(x^2)/x

You are forgetting to apply the chain rule in the numerator...you could also multiply by x/x to get the same result of 0.

3. ## Re: limit as x goes to zero of sin(x^2)/x

so for the the above it should be $cos(x^2)*2x/1$ which would be 1*0/1=0

multiplying by x/x we would get $xsin(x^2)/x^2$ which is equal is also equal to zero too but how? I feel like i'm missing some obvious step for version where we multiply by x/x

also is $sin(x^2)/x=sin^2x/x=(sinx)^2/x$ ? or is it just the last two that are the same?

4. ## Re: limit as x goes to zero of sin(x^2)/x

Originally Posted by kingsolomonsgrave
so for the the above it should be $cos(x^2)*2x/1$ which would be 1*0/1=0

multiplying by x/x we would get $xsin(x^2)/x^2$ which is equal is also equal to zero too but how? I feel like i'm missing some obvious step for version where we multiply by x/x

also is $sin(x^2)/x=sin^2x/x=(sinx)^2/x$ ? or is it just the last two that are the same.]?
\displaystyle \begin{align*} \lim_{x \to 0}\frac{\sin{\left(x^2\right)}}{x} &= \lim_{x \to 0}\frac{x\sin{\left(x^2\right)}}{x^2} \\ &= \lim_{x \to 0} x \cdot \lim_{x \to 0}\frac{\sin{\left(x^2\right)}}{x^2} \\ &= 0 \cdot 1 \\ &= 0 \end{align*}

Also, no, \displaystyle \begin{align*} \sin{\left(x^2\right)} \neq \sin^2{x} \end{align*}, rather \displaystyle \begin{align*} \sin^2{x} = \left(\sin{x}\right)^2 \end{align*}.

5. ## Re: limit as x goes to zero of sin(x^2)/x

We have:

$\lim_{x\to0}\frac{x\sin(x^2)}{x^2}=\lim_{x\to0}x \cdot \lim_{x\to0}\frac{\sin(x^2)}{x^2}$

For the second limit let $u=x^2$ and as $x\to0$ we have $u\to0$ so we may write:

$\lim_{x\to0}x\cdot\lim_{u\to0}\frac{\sin(u)}{u}=0 \cdot1=0$

I am assuming you have been given $\lim_{u\to0}\frac{\sin(u)}{u}=1$ to use for such limits.

6. ## Re: limit as x goes to zero of sin(x^2)/x

Thanks, yes we were given lim as x-> 0 sin(u)/u = 1

thanks much

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