calculate the resistance of 1 km long copper wire of radius 1mm resisitivity  of the copper is 1.72 *18 - 8 -ohm - meter 

Dear Student ,
We know that the resistance is given as

R = ρL/A

here,

ρ = 1.72 x 10^-8 ohm.m

L = 1000 m

A = πr² = π(0.5x10^-3)² = 7.85 x 10^-7 m²

so,

R = ( 1.72 x 10^-8 x 1000 ) / 7.85 x 10^-7 

thus, resistance of the wire will be

R = 21.91 $\Omega$

Regards