# U Substitution 1

1. Sep 12, 2007

So I have another U substitution.

$$\int sec^3(2x)tan(2x)$$ this one is a little tricky for me. I have tried letting u= sec2x and tanx and 2x.
2x definitley gets me nowhere. I may be mistaken on the others. I will recheck them.

I was also thinking of rewriting it as
$$\int sec^4(2x)sin(2x)$$

is the latter the better option?

Thanks,
Casey

2. Sep 12, 2007

### mattmns

Unless I am mistaken, u = sec2x should work.

3. Sep 12, 2007

Oh...one second, let me re-work this.

4. Sep 12, 2007

I must be missing something...

$$\int sec^32xtan2x$$ If u=sec2x du=2sec2xtan2x dx
$$\rightarrow \int u^3 tan2x$$

$$\int sec2xtanx*u^2 dx$$
=$$\frac{1}{2}\int u^2du$$

Thanks...Btw, how do you make a new line with Latex? I though it was \\

Last edited: Sep 12, 2007
5. Sep 12, 2007

### rocomath

your last 2 lines look funny to me, from what i have written down it should look like

$$\int\sec^{2}{2x}\sec{2x}\tan{2x}dx$$

subst. $$\sec{2x}tan{2x}dx$$ with $$du$$

$$\frac{1}{2} \int u^{2}du$$

your final integral looked good but the one b4 doesn't click with me. i also thought \\ starts a new line, it works sometimes, and others it doesn't. the \ doesn't even work great for me either.

6. Sep 12, 2007

I edited to fix..I forgot the tan2x where sec2xtan2x=du/2