Basic fraction manipulation functions? (neophyte question)

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Somewhere I thought I saw a sage function that splits fractions, e.g.

(a + b)/c to a/c + b/c

I haven't been able to find anything like this in the online documentation. If this exists, does anyone know the function name?

In general, I'm having a hard time finding documentation on basic expression manipulation. E.g. I've used expr.collect(x) but I can't find documentation on it. Similarly for expr.op[0], x.substitute(a == b) and simplify(expr) I'd appreciate any suggestions re finding documentation on these sort of operations.

thanks