Find the equivalent resistance between `A` & `B` in the given network.

Explanation

When hypothetical battery is connected to the given circuit the current distribution will be as shown.
`(1)/(R_(AB))=((R+3r))/(r(r+3R))+(1)/(r )`
`=(R+3r+r+3R)/(r(r+3R))`
`rArr R_(AB)=(r(r+3R))/(4(R+r))`