Another integration


by frozen7
Tags: integration
frozen7
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#1
Dec28-05, 08:10 AM
P: 167
[tex]\int \frac{1}{x^2 +4} [/tex] How to integrate this without knowing the derivatives of inverse function?
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arildno
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#2
Dec28-05, 08:14 AM
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If you don't know about an anti-derivative of the integrand, or are unable to transform your integrand in such a manner that an anti-derivative becomes apparent, then the fundamental theorem of calculus is of minor use to you in that particular case in the evaluation of your integral.
what
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#3
Dec28-05, 08:46 AM
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I think he/she is asking whether you need to know inverse trig derivatives/integrals to do this problem. You don't need to know the derivative of inverse trigonometric functions to do this problem, however you need to know about trig substitutions, if you can do trig substitutions then this integral can be done in two steps, one is a regular substitution that gets it in a form such as 1/(u^2 +1) and another trig substitution after that.

Pseudo Statistic
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#4
Dec28-05, 08:56 AM
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Another integration


Quote Quote by frozen7
[tex]\int \frac{1}{x^2 +4} [/tex] How to integrate this without knowing the derivatives of inverse function?
You can use some basic algebra and the method of partial fractions..
If you facter (x^2 + 4) you get (x + 2i)(x - 2i) where i is the complex number, sqrt(-1).
From there you can decompose 1/(x^2 + 4) into two fractions:

1/(x^2 + 4) = A/(x + 2i) + B/(x - 2i)

And you'll end up with a complex logarithm as the answer-- which will fit the definition of arctan, if I'm not mistaken. (Or something like that)

Either way plugging numbers in will still get you the same results as the real answer. :)
frozen7
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#5
Dec28-05, 09:00 AM
P: 167
Quote Quote by what
this integral can be done in two steps, one is a regular substitution that gets it in a form such as 1/(u^2 +1) and another trig substitution after that.
Could you explain further?
what
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#6
Dec28-05, 09:06 AM
P: 92
I'll assume you can do the first step, then use the trig substitution x = tan(θ). Remember after you have your answer you have to get it back in terms of x, your answer will be in terms of theta.

Edit: forgot a comma and a then, it might make it confusing.
VietDao29
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#7
Dec28-05, 09:34 AM
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Quote Quote by what
I'll assume you can do the first step, then use the trig substitution x = tan(θ). Remember after you have your answer you have to get it back in terms of x, your answer will be in terms of theta.
Edit: forgot a comma and a then, it might make it confusing.
Are you sure the substitution is [itex]x = \tan \theta[/itex]?
@frozen7:
You should look it up in your textbook or try here
So, what's the substitution? Is it [itex]x = \tan \theta[/itex]? Or what is it?
andrewchang
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#8
Dec28-05, 10:03 AM
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in the integral, you're missing the dx.

draw a triangle, it should help you see which values to substitute.
maverick6664
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#9
Dec28-05, 11:59 AM
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Quote Quote by frozen7
[tex]\int \frac{1}{x^2 +4} [/tex] How to integrate this without knowing the derivatives of inverse function?
If you know integral of inverse function it's easy.
At first [tex]\int \frac 1 {x^2+1} dx = \arctan(x)[/tex]

Now put [tex]u=2x[/tex] and you'll get

[tex] \frac 1 8 \int \frac 1 {u^2+1} du = 1/8 \arctan(2x)[/tex]
marlon
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#10
Dec28-05, 12:16 PM
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maverick, the substitution is not u=2x but u=x/2

[tex]du = \frac{dx}{2}[/tex]
[tex]\frac{1}{x^2+4} = \frac{1}{4(\frac{x^2}{4} + 1)} = \frac{1}{4( ( \frac{x}{2} )^2 + 1)}[/tex]

marlon
maverick6664
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#11
Dec28-05, 12:22 PM
P: 80
Quote Quote by marlon
maverick, the substitution is not u=2x but u=x/2
[tex]\frac{1}{x^2+4} = \frac{1}{4(\frac{x^2}{4} + 1)}[/tex]
marlon
oh my bad! Thank you! I'm a bit euphoric because my favorite female speed skater Tomomi Okazaki was chosen for Trino Olympic

Then correction:

[tex] \frac 1 2 \int \frac 1 {u^2+1} du = \arctan(x/2)[/tex]

hm, this looks smarter.
marlon
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#12
Dec28-05, 12:24 PM
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Quote Quote by maverick6664
oh my bad! Thank you!
Then correction:
[tex] \frac 1 2 \int \frac 1 {u^2+1} du = \arctan(x/2)[/tex]
I don't wanna be whinning but the right hand side of your equation is not correct either. You are forgetting something :)

marlon
maverick6664
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#13
Dec28-05, 12:29 PM
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Quote Quote by marlon
I don't wanna be whinning but the right hand side of your equation is not correct either. You are forgetting something :)
marlon
ooops!!! lol

[tex]1/2 \arctan(x/2)[/tex]

maybe i need sleep. It's 3:30am.
marlon
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#14
Dec28-05, 12:34 PM
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Quote Quote by maverick6664
maybe i need sleep. It's 3:30am.
Aha, that happens to me too.

Here in Western Europe it's 19.30 pm and i am looking foreward to attend the SAW2 premiere tonight.

OOHH YES, THERE WILL BE BLOOD

Sleep well :)

marlon
maverick6664
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#15
Dec28-05, 04:46 PM
P: 80
Quote Quote by marlon
Aha, that happens to me too.
Here in Western Europe it's 19.30 pm and i am looking foreward to attend the SAW2 premiere tonight.
OOHH YES, THERE WILL BE BLOOD
Sleep well :)
marlon
Got up now..but it's still 7:40am..I'll sleep again.. New Years day my family will pay first visit to a nearby shrine and my father's grave, and from Jan.2 I'll be "Home alone" because my wife and daughters will be in my wife's home town for nearly a week. Still sleepy... nite... (looks like I'm hijacking this thread.. this is a users' forum......) zzzzz...
Pseudo Statistic
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#16
Dec29-05, 01:45 AM
P: 390
What's up with all of the arctans? Why not be frank with the function and do what you gotta do..... use partial fractions!?
d_leet
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#17
Dec29-05, 02:46 AM
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Quote Quote by Pseudo Statistic
What's up with all of the arctans? Why not be frank with the function and do what you gotta do..... use partial fractions!?
Because
[tex]\int \frac 1 {u^2+a^2} du = \frac 1 {a} \arctan( \frac u {a} ) + C[/tex]
marlon
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#18
Dec29-05, 07:08 AM
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Quote Quote by Pseudo Statistic
What's up with all of the arctans? Why not be frank with the function and do what you gotta do..... use partial fractions!?
Because than you would be needing complex functions. That's against the rules if the calculus must remain "real"...

[tex]\frac{1}{x^2+a^2} = \frac{1}{x+ia}\frac{1}{x-ia} [/tex]

marlon


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