Trying to determine the output of a RC-filter with load
You can rearrange the circuit:







simulate this circuit &ndash; Schematic created using CircuitLab

But using Thévenin equivalence, you have this:







simulate this circuit

Which is a standard RC circuit with $$ V'_{in} = {R_2\over{R_1+R_2}}V_{in} $$ and $$ R_p = R_1||R_2 = {R_1R_2\over{R_1+R_2}}$$