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Reduction formula, integration problem

by JFonseka
Tags: formula, integration, reduction
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JFonseka
#1
Oct16-08, 04:46 AM
P: 117
1. The problem statement, all variables and given/known data

Part of an example of using reduction formula, I won't post the whole question as I get most of it, just at the very end, magical things happen with the working and things disappear, as they usually do with integration:

The definite integral goes from 0 to pi/4

=>[tex]\int[/tex] tan[tex]^{n-2}[/tex] x sec[tex]^{2}[/tex] x dx
=>tan[tex]^{n-1} x/n -1[/tex]

3. The attempt at a solution

My question is, what happened to the sec[tex]^{2}[/tex] x in the integration process?
I see the tan got integrated, but I can't figure out how sec disappears
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Hootenanny
#2
Oct16-08, 04:58 AM
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Notice that

[tex]\frac{d}{dx}\tan x = sec^2x[/tex]

[tex]\Rightarrow dx = \frac{d\left(\tan x\right)}{\sec^2 x}[/tex]
JFonseka
#3
Oct16-08, 05:09 AM
P: 117
Ah...I see, so that's how the sec^2 x gets canceled out.

Thanks hootenanny!

Hootenanny
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Oct16-08, 05:23 AM
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Reduction formula, integration problem

Quote Quote by JFonseka View Post
Ah...I see, so that's how the sec^2 x gets canceled out.

Thanks hootenanny!
A pleasure.
Mark44
#5
Oct16-08, 09:41 AM
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Since the integral is a definite integral, you should end up with a number, or at least an expression that doesn't involve n. Your final expression can easily be evaluated at both limits of integration.

As a minor point, you're using an "implies" symbol (==>) incorrectly. The first integral doesn't "imply" the second; it's equal to it.
Mark44
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Oct16-08, 09:46 AM
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Quote Quote by Hootenanny View Post
A pleasure.
It probably has been asked before, but what's the English equivalent of your signature line? I think I understand a few of the words.

disce quasi semper victurus vive quasi cras moriturus

speak? almost always of victory? live almost ?? (you?) die

Thanks
Hootenanny
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Oct16-08, 11:39 AM
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Quote Quote by Mark44 View Post
disce quasi semper victurus vive quasi cras moriturus

speak? almost always of victory? live almost ?? (you?) die
I'm impressed! You're quite close to the literal translation, but the meaning is

"Learn as if you were going to live forever, live as if you were going to die tomorrow".


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