$$\int\frac{dx}{\sqrt{x^2-81}}$$Tried $\cos u,\sin u,\tan u$ substitutions but nothing fit. :|

Question

anonymous · Answer

Woops, $\sec u$ substitution, my bad.

anonymous · Answer

But what do I do about$$\int\frac{1}{\tan u}$$

anonymous · Answer

i don't think this is doable with trig-substitution.  i don't know.  :(  sorry.

anonymous · Answer

It's the first problem in my textbook...

anonymous · Answer

what calc is this?  maybe find it numerically?

anonymous · Answer

Calc2, non-numerical

anonymous · Answer

maybe @directrix , @.Sam. , @Callisto, can help?

callisto · Answer

use sec theta

anonymous · Answer

Yeah, I just figured it out.

.sam. · Answer

$$x=\sec \theta$$

anonymous · Answer

Can I ask a further question?

anonymous · Answer

Or shoudl I create a new question?

.sam. · Answer

yeah, create new one

callisto · Answer

x=9sec theta ... not sec theta

anonymous · Answer

I know, it's the general hint that counts.

anonymous · Answer

.sam. · Answer

$$\sqrt{x^{2}-a^{2}}~~,x=asec \theta$$

anonymous · Answer

Since I solved it, I think it's safe to say the hint worked. :)