How to integrate int sin(ln(x)) ?

  • Thread starter Shaybay92
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In summary, the conversation is about solving the integral of sin(lnx) dx and using the substitution method with u=lnx. The person attempts to solve it but struggles with leaving the x in the integral. They eventually realize that x can be substituted with e^u and can then use integration by parts. The conversation also touches on the concept of inverse functions and clarifies that e^(lnx) does not cancel to 1.
  • #1
Shaybay92
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Homework Statement



I am having trouble solving the following:

[tex]\int[/tex]sin(lnx) dx


The Attempt at a Solution



I let u = ln x but this makes xdu = dx so I am left with

[tex]\int[/tex]x sinu du
 
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  • #2


That's the substitution you want to use,but you can't leave the x in there. If u=lnx, then x is?
 
  • #3


Would x = u2?

[tex]\int[/tex]ue sin u du

Then do you do it by parts or something?
 
  • #4


How on Earth did you get that? Can you solve [itex]u=\ln x[/itex] for x?
 
  • #5


Oops I meant e^u, is that right?
 
  • #6


Yes that's correct. You can now use integration by parts, twice.
 
  • #7


does e(lnx) cancel to 1?
 
  • #8


No and you should really know that at this stage! If a function has an inverse, [itex]f^{-1}(x)[/itex], then [itex]f^{-1}(f(x))=f(f^{-1}(x))=x[/itex]. So what is [itex]e^{\ln x}[/itex] and [itex]\ln e^x[/itex]?
 

Related to How to integrate int sin(ln(x)) ?

1. Can you explain the process of integrating int sin(ln(x))?

Integrating int sin(ln(x)) involves using the substitution method, where you substitute u=ln(x) and du=dx/x to turn the integral into int sin(u)du. From there, you can use the integration by parts method to solve the integral.

2. Is there a specific formula for integrating int sin(ln(x))?

No, there is no specific formula for integrating int sin(ln(x)). It requires using the substitution and integration by parts methods to solve the integral.

3. Can you give an example of integrating int sin(ln(x))?

Sure, let's say we have the integral int sin(ln(x))dx. We can substitute u=ln(x) to get int sin(u)du. Then, using integration by parts with u=sin(u) and dv=du, we get the final result of -cos(ln(x))+C.

4. Are there any special techniques for integrating int sin(ln(x))?

Yes, the substitution and integration by parts methods are commonly used to solve integrals involving trigonometric and logarithmic functions. Additionally, using trigonometric identities or rewriting the integral in a different form may also be helpful.

5. What are some common mistakes to avoid when integrating int sin(ln(x))?

One common mistake is forgetting to substitute for u=ln(x) and attempting to integrate the original integral. It is also important to pay attention to the limits of integration and make sure they are adjusted accordingly after the substitution. Additionally, care must be taken when using integration by parts to avoid errors in the calculation.

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