trig limit

**ironman2** · October 25th 2008, 09:06 AM

hi, i need urgent to solve this limit $\lim_{x \to \pi/6} \frac{2cosx-\sqrt3}{sin(\pi/6-x)}$

**Krizalid** · October 25th 2008, 09:07 AM

Put $z=\frac\pi6-x,$ remaining stuff is just a matter of algebra.

**ironman2** · October 25th 2008, 09:19 AM

can you do the whole solution please?

**Peritus** · October 25th 2008, 09:25 AM

L'Hospital

**Soroban** · October 25th 2008, 09:29 AM

Hello, ironman2!

Are you allowed to use L'Hopital?

$\lim_{x \to\frac{\pi}{6}} \frac{2\cos x-\sqrt3}{\sin(\frac{\pi}{6}-x)}$

Apply L'Hopital: . $\lim_{x\to\frac{\pi}{6}}\left[ \frac{-2\sin x}{-\cos(\frac{\pi}{6}-x)}\right] \;=\; \lim_{x\to\frac{\pi}{6}}\frac{2\sin x}{\cos(\frac{\pi}{6}-x)}$

. . $=\;\frac{2\sin\frac{\pi}{6}}{\cos(\frac{\pi}{6}-\frac{\pi}{6})} \;=\;\frac{2\cdot\frac{1}{2}}{\cos 0} \;=\;\frac{1}{1} \;=\;1$

**Krizalid** · October 25th 2008, 09:39 AM

Put $z=\frac\pi6-x$ and the limit becomes,

$\underset{z\to 0}{\mathop{\lim }}\,\frac{\sqrt{3}(\cos z-1)+\sin z}{\sin z}=\underset{u\to 0}{\mathop{\lim }}\,\frac{\dfrac{\sqrt{3}(\cos z-1)}{z}+\dfrac{\sin z}{z}}{\dfrac{\sin z}{z}}=1.$

**ironman2** · October 25th 2008, 01:04 PM

Soroban, first thank you very much. But it's one thing left, how did u changed numerator from 2cosx-sqrt3 to -2sinx? With other words how to apply this Hopital Rule (i didn't know for this since it was mentioned here). Can you explain with your words what did you do? Sorry for this ironic questions, but I'm just trying to learn, and I don't understand other literatures about this rule.

A little help would be highly appreciated.

p.s sorry for my bad english.

**mr fantastic** · October 25th 2008, 01:33 PM

Originally Posted by ironman2

Soroban, first thank you very much. But it's one thing left, how did u changed numerator from 2cosx-sqrt3 to -2sinx? With other words how to apply this Hopital Rule (i didn't know for this since it was mentioned here). Can you explain with your words what did you do? Sorry for this ironic questions, but I'm just trying to learn, and I don't understand other literatures about this rule.

A little help would be highly appreciated.

p.s sorry for my bad english.

If you have not learned l'Hopital's Rule then I doubt very much whoever gave you the question would expect you to use it. Therefore you should do it using Krizalid's suggestions.

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