How do I calculate the transfer function of this basic terminated RC filter?
There is a glitch in the book answer. The expression framed in red should have R'/R in the numerator not R'.
Remember also that Vout /Vin has no unit and the expression framed in red is in ohm.
This wont affect the book result for the cutoff frequency so it makes sense to mention this analogy.
And if you want to apply Thevenin do not forget to replace Vin by Vth also.