MHB Evaluating Limits using L'Hospital

[shamieh]

Need someone to check my work.lim t -> 0

$$\frac{e^{2t} - 1}{1 - cos(t)}$$

after I took the derivative twice

I got $$\frac{2}{0}$$ = undefined?

 

[MarkFL]

You can determine the limit does not exist after just one application of L'Hôpital's Rule. I would also look at the one-sided limits. What to you find?

Note: use the $\LaTeX$ code \lim_{t\to0} for your limit.

 

[shamieh]

Oh because the denominator has the cos and sin. that's what you are saying correct? How I could of known after 1 application of LHopsital...So is this the correct answer though? Does not exist? Or Should I put "Undefined"?

 

[MarkFL]

This is what I would write:

$$L=\lim_{t\to0}\frac{e^{2t}-1}{1-\cos(t)}$$

This is the indeterminate form $$\frac{0}{0}$$, so application of L'Hôpital's rule yields:

$$L=\lim_{t\to0}\frac{2e^{2t}}{\sin(t)}=\frac{2}{0}$$

This is undefined, so we want to look at the one-sided limits:

$$\lim_{t\to0^{-}}\frac{2e^{2t}}{\sin(t)}=-\infty$$

$$\lim_{t\to0^{+}}\frac{2e^{2t}}{\sin(t)}=\infty$$

Since:

$$\lim_{t\to0^{-}}\frac{2e^{2t}}{\sin(t)}\ne \lim_{t\to0^{+}}\frac{2e^{2t}}{\sin(t)}$$ we may conclude that the limit $L$ does not exist.