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do you really want us to go give you a mathematical solution for all cases you can think of?
* DC, what range?
* AC, what frequency, amplitude
* What values of the parts?
some approches for DC cases: (for DC cases you may omit C1)
* when V_IN is about 5V7 --> no current flow through D1, R2, R1 --> Ib is about zero.
* when V_IN < (5V7 - D1_forward_voltage) ... (= when D1 becomes conductive) calculate: V_R1 = (5V7 - V_in - V_d1) * R1 / (R1 + R2)
* now you have some new cases:
.. * when V_R1 < V_BE_th: then ib is about zero
.. * When V_R1 > V_BE_th: then consider V_R1 is about V_BE. Calculate I_R1. calculate anew I_R2, the difference between both is Ib.
It all depends a lot on device types, temperature and so on.