R5 and R6 are two resistors in series between two voltage sources. If the two voltages are not equal, the voltage at the center node formed by the two resistors can be calculated using Ohm's Law.
The corner frequency is calculated using the equation for the corner frequency of a single-pole R-C circuit. For the R, use the Thevenin equivalent or R5 and R6, which is the calculated value of R5 and R6 in parallel.
I am not able to understand why they have used R5 , because when I simulate the circuit taking out R5 , it still works the same , with little disturbances. What is the real need for using R5. Please help
R5, along with R6, c1, and pulses from v2 make up the bias voltage on the base of Q1. R5 and R6 act as a voltage divider.
R5 puts about 5 volts on the base of Q1 minus the voltage drop across it. R6 couples pulses from V2 to C1 which also biases Q1. The two sources add to produce final bias. If you remove R5, it should change the frequency but it will not turn Q1 off. You would need to remove R5 and R6 to turn off Q1 and stop the multivibrator from oscillating.