In this amplifier circuit we are using a voltage divider bias. Why can we consider that voltage produced by R2 is actually providing voltage to the base. What is R1 doing?
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R1 and R2 form a voltage divider between the supply and ground. The output of this divider provides the voltage and current to bias the transistor base.
This is a good place to do a Thevenin substitution. You can model the voltage divider as a fixed voltage source in series with a single resistance. The impedance of the Thevenin source, which is the resistance in series with the perfect voltage source, is the parallel combination of R1 and R2. To get the voltage of the fixed voltage source, look at the open circuit output of the divider, which is Vcc * R2(R1 + R2).
You haven't shown any numbers, so I'll leave the equations in symbolic form.