[tex]R_{AD} = (3R)||(R)||(3R) = \frac{3R*R*3R}{3R*R+3R*3R+R*3R} = \frac{3R}{5}[/tex]
[tex]R_{AB}=[(3R)||(R)]+(2R) = \frac{3R*R}{3R+R}+2R=\frac{3R}{4}+2R=\frac{11R}{4}[/tex]
[tex]\frac{R_{AD}}{R_{AB}} = \frac{3R}{5}*\frac{4}{11R} = \frac{12}{55}[/tex]