please give me the explanation of this statement "the equivalent resistance is greater than the greatest resistance in the series combination "   

as well as "the equivalent resistance is lesser than the least of all resistance in parallel combination"

pls explain me clearly

In case of resistors connected in series the resultant resistance of a series combination (equivalent resistance) is the sum of individual resistances.

So, if there are 3 resistors say R₁, R₂ and R₃ connected in series then the equivalent resistance (Rs) is given as 

R_s = R_{1 +} R₂ + R₃

Thus, we can see that the equivalent resistance is greater than all the resistances, ie., the value of equivalent resistor is greater than the resistor with the greatest value in the series combination.

 i.e. R_s > R₁ or R₂ or R₃

 

In case of parallel combination, the reciprocal of resultant resistance of a parallel combination of resistors (equivalent resistor) is the sum of reciprocals of individual resistances.

 

Thus, we can see that the equivalent resistance is lesser than all the resistances, ie., the value of equivalent resistor is lesser than the resistor with the greatest value in the series combination.

i.e. Rp < R₁ or R₂ or R₃