[tex]R=\rho*\frac{l}{S_1}(1)\\ d_2=2d_1\\r_1=\frac{d_1}{2}\\r_2=\frac{d_2}{2}=\frac{2d_1}{2}=d_1=2r_1\\ S_1=\pi r_1^2\\S_2=\pi r_2^2\\R'=\rho*\frac{l}{S_2} (2) \\(1):(2)=>\frac{R}{R'}=\frac{S_2}{S_1}= \frac{\pi r_2^2}{\pi r_1^2}=\frac{4r_1^2}{r_1^2}=4=>R'=\frac{R}{4}->a)[/tex]
