a wire of 9 ohm resistance having 30cm length is tripled o itself.what is its new resistance and why??

Initially,

Resistance, R = 9 Ω

Length, L = 30 cm = 0.3 m

Let A be the area of cross-section and ρ be the resistivity.

When the length is stretched to 3L, the area of cross-section should decrease in such a way that the volume of the wire remain constant.

So, Volume = AL = A^/(3L)

=> A^/ = A/3

Now,

The new resistance is,

R^/ = ρ(3L)/A^/ = ρ(3L)/(A/3) = 9R = (9)(9) = 81 Ω