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First, note that such a limit can be computed by hand and does not requires any computer.

If your goal is to get this computation done with Sage, you can have a look at the following documentation to see how to define symbolic variables (the parameter d in your case), how to define a symbolic function (y in your case) and how to compute the limit: https://doc.sagemath.org/html/en/reference/calculus/sage/calculus/calculus.html

You can also look at the following introduction: https://doc.sagemath.org/html/en/prep/Calculus.html

If you want some more help, you should ask more precise questions related to your research in solving this exercise, especially where you are locked.