integrals of secx and 1/cosx

**hmmmm** · December 7th 2008, 10:01 AM

why are the integrals of secx and 1/cosx not the same??

**galactus** · December 7th 2008, 10:04 AM

They are the same. Because sec(x)=1/cos(x)

What difference do you see?. Please explain so I may know what you're getting at.

**hmmmm** · December 7th 2008, 10:11 AM

the integral of secx=

the integral of 1/cosx=

??

**galactus** · December 7th 2008, 10:15 AM

They are equivalent, just in a different form.

$tan(\frac{x}{2}+\frac{\pi}{4})=sec(x)+tan(x)$

Can you prove it?.

**Moo** · December 7th 2008, 10:24 AM

Originally Posted by hmmmm

the integral of secx=

the integral of 1/cosx=

??

And what if $\tan \left(\frac{ax}{2}+\frac \pi 4\right)=\sec(ax)+\tan(ax)$ ?

$\tan \left(\frac{ax}{2}+\frac \pi 4\right)=\frac{\tan \left(\frac{ax}{2}\right)+\tan \left(\frac \pi 4\right)}{1-\tan \left(\frac{ax}{2}\right)\tan \left(\frac \pi 4\right)}$
We know that $\tan \frac \pi 4=1$ . hence :

$\tan \left(\frac{ax}{2}+\frac \pi 4\right)=\frac{1+\tan \left(\frac{ax}{2}\right)}{1-\tan \left(\frac{ax}{2}\right)}$

Multiply by $\frac{1+\tan \left(\frac{ax}{2}\right)}{1+\tan \left(\frac{ax}{2}\right)}$ :

$=\frac{\left(1+\tan \left(\frac{ax}{2}\right)\right)^2}{1-\tan^2 \left(\frac{ax}{2}\right)}$

Now substitute $\tan^2t=\frac{1}{\cos^2t}-1$

Then multiply both numerator and denominator by cos² and use this formula : $2 \cos^2t-1=\cos(2t)$

This should help you go through the problem

**hmmmm** · December 7th 2008, 12:59 PM

thanks ive got it now

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thanks

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