Why are resistance connected in parallel?

With the help of a diagram derive the formula for the equivalent resistance of three resistances connected in parallels?

Why are resistance connected in series ?

With the help of a labelled circuit diagram derive a formula to find combined resistance (R) when two or more resistance are connected in parallel taking symbol potential difference (V) and current (I)

Three identical cells of emf 4V each and unknown internal resistances are connected in parallel.The combination is connected to 10 ohm resistor. If terminal voltage across the cells is 2V, find internal resistance of each cell.

Three identical cells of emf 4V each and unknown internal resistances are connected in parallel.The combination is connected to 10 ohm resistor. If terminal voltage across the cells is 2V, find internal resistance of each cell.

Three identical cells each of 2 V and unknow internal resistance are connected in parallel. This combination is connected to a 5 ohm resistor. IF the terminal voltage across the cell is 1.5 volt, what is the internal resistance of each cell?