P$\mathrm{.}9-10$ It is desired to construct uniform transmission lines using polyethylene $(.2\mathrm{.}29$ as the dielectric medium. Assuming negligible losses, $($a$)$ find the distance of separation for a $300(2)$ two$-$wire line, where the radius of the conducting wires is $0\mathrm{.}6($mm$)$; and $($b$)$ find the inner radius of the outer conductor for a $75($D$)$ coaxial line, where the radius of the center conductor is $0\mathrm{.}6($mm$).$