# integration

• Apr 7th 2006, 06:24 AM
them
integration
hi, is there any method of integrating a hyperbolic or trignometric function whos degree is 4, such as (tanx)^4, (cosechx)^4 WITHOUT resorting to reduction formulae??
• Apr 8th 2006, 06:21 AM
TD!
Sure, there just isn't a "golden recipe" which will be succesfull every time. You'll need to play arround with trig formulas. I'll do one as an example.

$\displaystyle \int {\tan ^4 x} dx = \int {\tan ^2 x\tan ^2 x} dx = \int {\tan ^2 x\left( {\sec ^2 x - 1} \right)} dx$

Now I'll split the integral in two. The first one is easy since the derivative of tan(x) is exactly secē(x), so:

$\displaystyle \int {\tan ^2 x\sec ^2 x} dx = \int {\tan ^2 x} d\left( {\tan x} \right) = \frac{{\tan ^3 x}}{3} + C$

For the second one; I convert to sin(x) and cos(x):

$\displaystyle \int { - \tan ^2 x} dx = - \int {\frac{{\sin ^2 x}}{{\cos ^2 x}}} dx = \int {\frac{{\cos ^2 x - 1}}{{\cos ^2 x}}} dx$

Splitting the integral in two again gives us simply:

$\displaystyle \int {dx} - \int {\frac{1}{{\cos ^2 x}}} dx = x - \tan x + C$

So we conclude, without reduction-formula:

$\displaystyle \int {\tan ^4 x} dx = \frac{{\tan ^3 x}}{3} - \tan x + x + C$
• Apr 8th 2006, 11:54 AM
them