Before we look at how to generate the textual form, let's correct a syntax error in your code; function application in Mathematica is indicated by square brackets, so
Sin(x + y) in your integral must replaced
Sin[x + y]. With this change your integral evaluates correctly:
We verify that the above is correct by taking the derivative with respect to $dx\,dy$.
D[-Sin[x + y], y, x]
Sin[x + y]
Now let's attend to converting the integral expression into a form that doesn't use special symbols. Best to make a copy of input cell when doing this (at least for your first time).
- Select the cell bracket at the right of the copied cell.
- Go to the Cell menu.
- Select Convert to from that menu.
- Click on Input Form on the pop-up sub-menu.
You should get
Integrate[Sin[x + y], y, x]
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