Improper integrals

  • Thread starter Chas3down
  • Start date
  • #1
60
0

Homework Statement



integral of 1/sqrt(9-x^2)
from 0 to 3



Homework Equations



///

The Attempt at a Solution


I integrate it correct to arcsin(x/3) from 0 to 3
Get the correct anwser of pi/2.

But there is another question, At which value of x in the integration region [0,3] does special care need to be taken with the integration? I understand at some point it goes from negatie to positive, but i tried 0,3,pi/2,pi.. none worked.. anyhelp?
 

Answers and Replies

  • #2
LCKurtz
Science Advisor
Homework Helper
Insights Author
Gold Member
9,559
770

Homework Statement



integral of 1/sqrt(9-x^2)
from 0 to 3



Homework Equations



///

The Attempt at a Solution


I integrate it correct to arcsin(x/3) from 0 to 3
Get the correct anwser of pi/2.

But there is another question, At which value of x in the integration region [0,3] does special care need to be taken with the integration? I understand at some point it goes from negatie to positive, but i tried 0,3,pi/2,pi.. none worked.. anyhelp?

What did you get when you put ##x=3## into the integrand?

And why do you say it goes from negative to positive?
 
  • #3
392
17

Homework Statement



integral of 1/sqrt(9-x^2)
from 0 to 3



Homework Equations



///

The Attempt at a Solution


I integrate it correct to arcsin(x/3) from 0 to 3
Get the correct anwser of pi/2.

But there is another question, At which value of x in the integration region [0,3] does special care need to be taken with the integration? I understand at some point it goes from negatie to positive, but i tried 0,3,pi/2,pi.. none worked.. anyhelp?

Check the domain of the original function to be integrated.
 
  • #4
60
0
I never had to do anything with the domain, it just worked.. But i guessed 0, pi/2 and 3. I thought it was be pi/2 because thats where it goes from neg to pos.
 
  • #5
392
17
I never had to do anything with the domain, it just worked.. But i guessed 0, pi/2 and 3. I thought it was be pi/2 because thats where it goes from neg to pos.

That's because you have been doing "proper" integrals up to this point. Improper integrals involve integrating across a point where the function is not defined. In this case the the function is not defined at x = __. The normal procedure is to introduce a variable for that number and take the limit as a approaches that number.

In this case, arcsin is defined on [0,1]. But the original function is not defined on [0,3].
 
  • #6
LCKurtz
Science Advisor
Homework Helper
Insights Author
Gold Member
9,559
770
What did you get when you put ##x=3## into the integrand?

And why do you say it goes from negative to positive?

I never had to do anything with the domain, it just worked.. But i guessed 0, pi/2 and 3. I thought it was be pi/2 because thats where it goes from neg to pos.

Try answering my two questions.
 

Related Threads on Improper integrals

  • Last Post
Replies
4
Views
810
  • Last Post
Replies
3
Views
1K
  • Last Post
Replies
13
Views
3K
  • Last Post
Replies
7
Views
2K
  • Last Post
Replies
5
Views
932
  • Last Post
Replies
2
Views
1K
  • Last Post
Replies
3
Views
664
  • Last Post
Replies
3
Views
2K
  • Last Post
Replies
3
Views
926
  • Last Post
Replies
6
Views
2K
Top