Quick integration by substitiution question

  • Thread starter trap101
  • Start date
  • Tags
    Integration
In summary: In this case you can expand to ##x^4-2x^2+1##. Then you can integrate term wise. First term gives ##\frac{x^5}{5}##, second term gives you ##-\frac{2x^3}{3}## and the third term gives you x. Plugging in the bounds you get ##\frac{0}{5}-\frac{4}{3}+0-(-\frac{2}{3})+0-0 = \frac{2}{3}##.In summary, the conversation discusses the process of integrating the function [3(1-x2)2]/4 from -1 to 1 and the use of substitution and expansion to
  • #1
trap101
342
0
integrate from (-1 to 1): [3(1-x2)2]/4 dx


so I'm having a little issue with the substitution for this. I'm going to let u = 1-x2, which
can give me x2 = 1-u, which I was going to put into my original equation and integrate. But I'm having issues finding the du to match with it.
 
Physics news on Phys.org
  • #2
trap101 said:
integrate from (-1 to 1): [3(1-x2)2]/4 dxso I'm having a little issue with the substitution for this. I'm going to let u = 1-x2, which
can give me x2 = 1-u, which I was going to put into my original equation and integrate. But I'm having issues finding the du to match with it.

In this case you should actually expand to make the integrand easier and factor out the (3/4) :

##(1-x^2)^2 = x^4 - 2x^2 + 1##
 
  • #3
Zondrina said:
In this case you should actually expand to make the integrand easier and factor out the (3/4) :

##(1-x^2)^2 = x^4 - 2x^2 + 1##

dang, your right. But now say I forgot that on a test. Is there anything I could do in other form to alleviate the situation, because I'm having a nightmare trying to figure something out.
 
  • #4
trap101 said:
dang, your right. But now say I forgot that on a test. Is there anything I could do in other form to alleviate the situation, because I'm having a nightmare trying to figure something out.

Take some deep breaths and ignore the problem for a few seconds, it will calm you down so you can think more clearly.

If you notice any obvious simplification you can do to the integrand before you actually evaluate it, then do it, sometimes a substitution isn't even required.
 
  • #5
Zondrina is giving you the right idea here. Usually when you get stuck on these kind of problems you just have to stop and think: Where can I go with this? in your attempt you find that du=-2xdx, but there is no multiple of x in your integrand. This is problematic as you need to integrate with respect to u so you can't really take things further. What are your other options. One that almost instantly should come to mind with lower powers of polynomial terms like this one is just to expand. Once you expand a polynomial you can integrate term wise.
 

FAQ: Quick integration by substitiution question

What is Quick Integration by Substitution?

Quick integration by substitution is a method used in calculus to find the integral of a function by substituting a new variable for the original variable in the integral.

When should I use Quick Integration by Substitution?

This method should be used when the integral contains a function within a function, or when the integral can be simplified by substituting a new variable.

How do I perform Quick Integration by Substitution?

To perform quick integration by substitution, follow these steps:

1. Identify the inner function within the integral

2. Substitute a new variable for the inner function

3. Rewrite the integral in terms of the new variable

4. Solve the integral using traditional integration techniques

5. Substitute the original variable back into the solution

What is the benefit of using Quick Integration by Substitution?

The benefit of using this method is that it simplifies the integral and makes it easier to solve using traditional integration techniques.

Are there any tips for mastering Quick Integration by Substitution?

To master quick integration by substitution, it is important to practice and become familiar with identifying the inner function and choosing the appropriate substitution. It is also helpful to review and understand traditional integration techniques to apply them correctly after substitution.

Similar threads

Replies
8
Views
1K
Replies
22
Views
2K
Replies
15
Views
1K
Replies
10
Views
1K
Replies
27
Views
2K
Back
Top