# Converting single integral to double integral

## Homework Statement

What is the procedure in converting that single integral, dividing it into parts, and making it a double integral?

And also, how Venus took $\sin(x)$ and brought it inside the first integral, and interchanging the integrals?

What is the criterion?

I am very interested in this.

Thanks!

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## The Attempt at a Solution

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I am looking for a general method, thats all.