Question

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

Answer 1

Woops, \(\sec u\) substitution, my bad.

Answer 2

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

Answer 3

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

Answer 4

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

Answer 5

what calc is this? maybe find it numerically?

Answer 6

Calc2, non-numerical

Answer 7

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

Answer 8

use sec theta

Answer 9

Yeah, I just figured it out.

Answer 10

\[x=\sec \theta\]

Answer 11

Can I ask a further question?

Answer 12

Or shoudl I create a new question?

Answer 13

yeah, create new one

Answer 14

x=9sec theta ... not sec theta

Answer 15

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

Answer 16

New question. http://openstudy.com/study#/updates/4f702f3ce4b0eb858772ee32

Answer 17

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

Answer 18

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