Efficient U-Substitution for (x^2)(sinx)/(1+x^6)

In summary, the conversation is discussing an integration problem involving the expression [ ( x^2 ) ( sinx ) ] / (1 + x^6). One person suggests using a substitution u = cos x and integrating by parts, while another person advises to use a different substitution w = x^3. The conversation also emphasizes the importance of providing complete information and showing effort when seeking help.
  • #1
nameVoid
241
0
[ ( x^2 ) ( sinx ) ] / (1 + x^6)
 
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  • #2
17.4
 
  • #3
Cristo, I get 42. You might have forgotten to multiply by Hooker's constant (= 2.413793103).
 
  • #4
Don't listen to these clowns. The answer you seek is

[ ( u^2 ) ( sinu ) ] / (1 + u^6)
 
  • #5
jbunniii said:
Don't listen to these clowns. The answer you seek is

[ ( u^2 ) ( sinu ) ] / (1 + u^6)

That's an extremely useful substitution in those cases where you find the given variable esthetically displeasing for some reason.

For all other cases, not so much.
 
  • #6
nameVoid,
If you're still out there, you would have gotten more serious (and helpful) responses if you had given us the complete problem. We can infer that this is an integration problem, although there was no indication of that in what you wrote. Also, if you want help, show us what you've tried.

The integral looks to me like it could be done using integration by parts in this way:
u = cos x
[tex]dv = \frac{x^2 dx}{1 + x^6}[/tex]

To find v, you'll have to integrate dv, which involves a substitution w = x3, dw = 3x2dx. I haven't worked it out, but this is what I would try first.
 

Related to Efficient U-Substitution for (x^2)(sinx)/(1+x^6)

1. How do I perform efficient U-substitution for (x^2)(sinx)/(1+x^6)?

To perform efficient U-substitution for (x^2)(sinx)/(1+x^6), you first need to identify the part of the expression that can be substituted with a single variable, known as "u". In this case, the expression (1+x^6) can be substituted with u. Then, you need to find the derivative of u with respect to x, which is du/dx = 6x^5. Finally, you can rewrite the original expression in terms of u and du, and use the substitution formula to solve for the integral.

2. What is the substitution formula for U-substitution?

The substitution formula for U-substitution is: ∫f(x) dx = ∫f(u) (du/dx) dx. This formula allows you to substitute a variable, u, and its derivative, du/dx, in place of a more complex expression in order to simplify the integral.

3. Can U-substitution be used for all integrals?

No, U-substitution can only be used for integrals where the integrand (the function being integrated) can be rewritten in terms of a single variable and its derivative. This is why it is important to carefully choose the substitution variable, u, in order to make the integral simpler.

4. Are there any specific rules to follow when using U-substitution?

Yes, there are a few rules to follow when using U-substitution. First, you must make sure that the expression inside the integral can be rewritten in terms of a single variable and its derivative. Second, you must make sure to adjust the limits of integration when substituting u for the original variable. Third, you must use the substitution formula to rewrite the integral in terms of u and du. Lastly, you must remember to substitute back in the original variable at the end to get the final answer.

5. Can U-substitution be used for definite integrals?

Yes, U-substitution can be used for both indefinite and definite integrals. However, when using U-substitution for a definite integral, it is important to adjust the limits of integration to match the substitution. This is done by substituting the original variable (x) into the limits and then replacing it with u.

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