SE BoardHow Can I Solve This Trigonometric Limit Problem?

[MacLaddy1]

Hello math helpers and others. Before I ask my question I would like to say that I appreciate everyone's help on these boards, and I hope that I will not be too large of a nuisance in the future. I am in my first calculus class, and it appears that I am going to need a lot of help. I was a member of the previous forum, but I didn't post often, so I hope there isn't any type of limit.

Here's my question.$$\displaystyle\lim_{\theta\rightarrow 0} {\frac{\sec(\theta)-1}{\theta}}/$$

Um, really don't know where to go from here. Should I expand that numerator to be (1/cos(Θ)) - 1? I know the basic rules of sinx/x = 1, and cosx-1/x = 0, and how to simplify simple things like sin2x/x, but I can't seem to get a start on this one.

Any help would be greatly appreciated. Also, I usually prefer to use Latex, but my typical tags aren&#039;t working, and $$ centers it. Any advice on getting an in line equation, so I don&#039;t just have to type sin(x)/x?<br /> <br /> Thanks again,<br /> Mac<br /> <br /> *EDIT* I think I figured out the LaTex

 

[alexmahone]

Should I expand that numerator to be (1/cos(Θ)) - 1?

Yes. And then use the identity $\displaystyle 1-\cos\theta=2\sin^2\frac{\theta}{2}$.

 

[MacLaddy1]

I'm sorry, I am probably just a bit tired, but I don't understand how you go from \(\frac{\frac{1}{cos{\theta}}-1}{\theta}\) to \((1-\cos{\theta})\) identity. I'll look at this more tomorrow when I am a bit clearer, but if you could elaborate some I would appreciate it.

Thanks again,
Mac

 

[Prove It]

I'm sorry, I am probably just a bit tired, but I don't understand how you go from \(\frac{\frac{1}{cos{\theta}}-1}{\theta}\) to \((1-\cos{\theta})\) identity. I'll look at this more tomorrow when I am a bit clearer, but if you could elaborate some I would appreciate it.

Thanks again,
Mac

\[ \displaystyle \begin{align*} \frac{\frac{1}{\cos{\theta}} - 1}{\theta} &= \frac{\frac{1 - \cos{\theta}}{\cos{\theta}}}{\theta} \\ &= \frac{1 - \cos{\theta}}{\theta\cos{\theta}} \\ &= \frac{1 - \cos{\theta}}{\theta} \cdot \frac{1}{\cos{\theta}} \end{align*} \]

I'm sure you know that the limit of a product is equal to the product of the limits...

 

[MacLaddy1]

Okay, thanks guys. I am following you now. The step I was missing was multiplying all by cos(Θ). It simplifies to 0 * 1 in the bottom.

Very much appreciated, thank you.

 

[CaptainBlack]

I'm sure you know that the limit of a product is equal to the product of the limits...

As long as both exist.

\[ \lim_{x \to 0}\left( x \times (1/x)\right) =1 \ne \left(\lim_{x \to 0} (x) \right) \left(\lim_{x \to 0} (1/x) \right) \text{ which is undefined }\]

CB