how to find RC time constant of series parallel circuit?

[love_electronic]

Its a silly question
But i want to know how to calculate the time constant of the following circuit

 

[LvW]

A simple method is to assume that the capacitor is charged - and to determine the resistance for the discharging process.
Assuming that R1 is connected to an ideal voltage source (zero ohms source resistance) the resulting time constant is T=(R1||R2)*C.

As another method, you can find the transfer function. If it is written in the classical form (denominator: 1+jw*T) you can directly identify the time constant.

 

[ferdem]

Thevenin theorem should work here. You can find equivalent circuit seen by the capactior. You will see Req=(R1+Rsource)//R2 then T=Req*C.