# Math Help - secxtanx=1?

1. ## secxtanx=1?

Can I use identities to make sec(x)tan(x)=1?

Moderator edit: This question is not relevant to the Calculus subforum. However, the relevance of this thread to the Calculus subforum only becomes clear with post #3.

2. Originally Posted by dbakeg00
Can I use identities to make sec(x)tan(x)=1?
What do you mean by 'make'?

Do you mean solve $\sec(x)\tan(x)=1$ for $x~?$

3. This is the wrong forum for this but my original problem is $\displaystyle\int\frac{secxtanx}{9+4sec^2x}dx$

So I used the integral identity $\displaystyle\int\frac{1}{a^2+x^2}dx=\frac{1}{a}ta n^{-1}\frac{x}{a}+c$

Then I rewrote the problem $\displaystyle\int\ secxtanx*\frac{1}{3^3+2^2sec^2x}dx$

$\displaystyle\int secxtanx*\frac{1}{\frac{1}{4}[(\frac{3}{2})^2+sec^2x]}dx$

So, if i could find a way to transform that secxtanx, then I would be home free...is that clear?

4. You do know that $\dfrac{{d\sec (x)}}{{dx}} = \sec (x)\tan (x)~?$

5. Originally Posted by Plato
You do know that $\dfrac{{d\sec (x)}}{{dx}} = \sec (x)\tan (x)~?$
I do..and maybe I'm missing something because I'm still new to integrals but the answer given in the book is:

$\displaystyle\frac{1}{6}tan^{-1}(\frac{2secx}{3})+c$

which is what I come up with when I integrate this part

$\displaystyle\frac{1}{\frac{1}{4}[(\frac{3}{2})^2+sec^2x]}dx$

so I had the secxtanx left over and didnt know what to do with it

6. $\displaystyle \int {\frac{{\sec (x)\tan (x)}}
{{9 + 4\sec ^2 (x)}}dx = \frac{1}
{4}\int {\frac{{\sec (x)\tan (x)}}
{{\left( {\frac{3}
{2}} \right)^2 + \sec ^2 (x)}}dx = \frac{1}
{4}\left( {\frac{2}
{3}} \right)\arctan \left( {\frac{{2x}}
{3}} \right)} }
$

7. So what happened to the sec(x)tan(x)? shouldn't it be sec(x)?

8. Originally Posted by dbakeg00
So what happened to the sec(x)tan(x)? shouldn't it be sec(x)?
Let $u=\sec(x)$ then $\displaystyle \[\frac{1}
{4}\int {\frac{{\sec (x)\tan (x)dx}}{{\left( {\frac{3}
{2}} \right)^2 + \sec ^2 (x)}}} = \frac{1}{4}\int {\frac{{du}}
{{\left( {\frac{3}{2}} \right)^2 + u^2 }}}$

9. there goes the light bulb! Thanks, I really appreciate it!