As shown in the figure, the hybrid connection mode is changed, but the positions of the lines are rearranged according to the topological relationship.

The equivalent resistance R3∑R4 of R3 and R4 in parallel is 3/2 = 1.5 (ohm), which is connected in series with R 1, and the equivalent resistance R is 3+ 1.5 = 4.5 (ohm).

The equivalent resistance between AB is the parallel resistance of R and R2, and Rab = R× R2/(R+R2) = 4.5× 3/(4.5+3) =1.8 (ohm).