Question

with the help of a circuit diagram, deduce the equivalent resistance of two resistance connected in series

Answer 1

Two resistors of resistance R1 and R2 are connected in series. Let I be the current through the circuit. The current through each resistor is also I. The two resistors joined in series is replaced by an equivalent single resistor of resistance R such that the potential difference V across it, and the current I through the circuit remains same. 

Answer 2

The voltage drop is the same across each parallel branch. The sum of the current in each individual branch is equal to the current outside the branches. The equivalent or overall resistance of the collection of resistors is given by the equation 1/Req = 1/R1 + 1/R2 + 1/R3 ...