If a copper wire is stretched to make its radius decrease by 0.1%, then percentage increase in resistance is approximately

In case of stretching volume of wire is assumed to remain unchanged. ∴lA=constant and L=cA∴R=ρ.lA=ρ.CA2=ρ.C(Ar2)2=R∝ 1r4∴ΔRR=−4Δrr×100%=(−4)×(−0.1%)=+0.4%