Integrate sin(y)/(x+y): Tips to Solve Double Integral

In summary, the person was integrating sin(y)/(x+y) dx between 0 and y, but was having trouble with the integration of the function in the first integral. They found the solution by entering the bounds and simplifying the problem.
  • #1
BananaMan
41
0
i have a double integral to reverse the order of and then integrate, i have reversed the order fine, however i am VERY stuck on the integration of the function in the first integral

integrate sin(y)/(x+y) dx between 0 and y

any pointers greately appreciated
 
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  • #2
You're integrating wrt x, right? So sin(y) is a contant as far as the integration is concerned.
 
  • #3
so it would just be sin(y) * integral 1/(x+y)

therefore sin(y) * ln(x+y)

if so I am going to feel like a fool :P
 
  • #4
if this is the case i have now integrated it and subbed for (y) (from the limits) to get sin(y)*ln(2y) but i must now integrate that function wrt y, which is even harder than the first function >.< and i am very stuck, maybe integration by parts here?
 
  • #5
Don't feel like a fool; the thing is so. Post the bounds so we can help...
 
  • #6
ok the original integral was between y and 0 hence getting sin(y)*ln(2y)

now the integral is that function I've just typed between pi/2 and 0 dy

im muchos stuck
 
  • #7
[tex] -\int_{0}^{\frac{\pi}{2}} \sin y \ \ln 2y \ dy =-\left[ \mbox{Ci}\left( \frac{1}{2}\pi \right) +\ln 2-\gamma\right][/tex].

Daniel.
 
  • #8
The bounds on the first integral are y and 0 right? So just enter the bounds correctly. Don't skip too many steps or you might miss a simplification:
[tex]\ln(x+y)|^{x=y}_{x=0}=\ln(2y)-\ln(y)=\ln(\frac{2y}{y})=\ln2[/tex]

Which doesn't even depend on y, so the integral is easy as pie.
 
  • #9
Galileo said:
The bounds on the first integral are y and 0 right? So just enter the bounds correctly. Don't skip too many steps or you might miss a simplification:
[tex]\ln(x+y)|^{x=y}_{x=0}=\ln(2y)-\ln(y)=\ln(\frac{2y}{y})=\ln2[/tex]

Which doesn't even depend on y, so the integral is easy as pie.

thanks :)

i just forgot to separate the variables and made life hard on myself
 

Related to Integrate sin(y)/(x+y): Tips to Solve Double Integral

1. How do I set up the double integral for sin(y)/(x+y)?

To set up the double integral for sin(y)/(x+y), you need to first determine the limits of integration for both x and y. This can be done by looking at the given bounds of the region of integration or by graphing the function. Once you have the limits, you can set up the integral as ∫∫sin(y)/(x+y) dy dx.

2. What is the order of integration for solving this double integral?

The order of integration for solving this double integral can be either dy dx or dx dy, since both orders will result in the same answer. It is usually more convenient to integrate with respect to the inner variable first, so in this case, it would be dy dx.

3. How do I handle the singularity at x=-y?

When x=-y, the denominator (x+y) becomes 0, which results in a singularity. To handle this, you can split the integral into two parts - one where x+y>0 and one where x+y<0. This will ensure that the integral is well-behaved and can be evaluated.

4. Can I use a change of variables to simplify the integral?

Yes, you can use a change of variables to simplify the integral. One common change of variables is to let u=x+y and v=x-y. This will transform the integral into a simpler form, which can then be evaluated using the new limits of integration.

5. What are some tips for solving this double integral?

Some tips for solving this double integral include: first, carefully setting up the limits of integration; then, using a change of variables to simplify the integral; and finally, checking for any singularities or special cases that may require splitting the integral into multiple parts. It is also important to pay attention to the order of integration and to take your time in evaluating the integral step by step.

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