# What is the rationale of solving SDE by Ito's formula?

When solving a SDE by Ito's formula, we have to find a function $f(t, X_t)$ of index $t$ and the process $X$ to be solved for.

1. I was wondering what is the criterion of choosing $f$? Is it to make the integrands not contain $X$?
2. How is $f$ chosen in principle?

Thanks and regards!

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