Question

derive the expression for resultant resistance when 3 resistors are connected (i) series(ii)parallel

Answer 1

For series connection of the resistors,

=> R = R1 + R2 + R3

For parallel Resistance,

=> 1/R = 1/R1 + 1/R2 + 1/R3

Answer 2

Do not go by formula .Go through concepts.

In series connection we add resistances.So,
For series connection resultant resistance=R1+R2+R3.
In parallel we add the inverses to get the inverse of resultant resistance.
So for parallel combination,
1/resultant resistance=1/R1+1/R2+1/R3.
If you will solve it,you will get,
Resultant resistance=(R1.R2.R3)/(R2R3+R3R1+R1R2)

Thanks.
Tripathy.