The graph of  $\mathcal{l}$n(R/R₀) versus $\mathcal{l}$nA (R = radius of a nucleus and A = its mass number) is–

Graph between log  $\frac{R}{R_0}$ and log_eA is always a straight line. 
 R = R₀ A^1/3 
$\frac{R}{R_0}$   = A^1/3  
 log_e   $\frac{R}{R_0}=\frac{1}{3}$log_eA