Question

To determine the equivalent resistance of two resistors when connected in parallel practical

Answer 1

add the reciprocal of individual resistance meaning take (1/Ra + 1/Rb)

Answer 2

Let the resistances of the resistors be "R₁" and "R₂".
Let the total resistance be "R"

The current , I ,  passing through each of them is different.
Let I₁ be the current passing through R₁ and I₂ be the  current passing through R₂

Total current = I
I = I₁ + I₂

P.d (potential difference) across both the resistors is same

We know that , by Ohm's law :-

I  = V/R

so ,

I₁ = V/R₁
I₂  V/R₂

so , 
total current would be :-

I = I₁ + I₂

that is ,

V/R = V/R₁ + V/R₂

cancelling "V" from both sides ,we get :-

1/R = 1/R₁ + 1/R₂

hence , 
total resistance of the resistors connected in series is the reciprocal of their individual resistances.