Derive R - pl/A.

Resistance of a conductor R is directly proportional to its length L at constant temperature and cross-sectional area

R ∝ L ----equation 1

Resistance R of a conductor is inversely proportional to its cross-sectional area A at constant temperature and length.

R ∝ ----- equation 2

Combining equation 1 and 2 we get-

So

Where ρ is proportionality constant and is called specific resistance or resistivity.