A wire of radius r has resistance r if it is stretched to a radius of 3r 4 its resistance becomes
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R = rho .L/A
A = pi (r^2)
when radius =3r/4 its resistance = R/r
where r is the required resistance
R/r = (radius.1/radius.2)^4
R/r = (3r/r)4 /r^4
R/r =(81r^4 / 4^4) /r^4
R/r =81r^4 / 256(r^4)
R/r =81/256
r => 256R/81
Guess its right
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