The blogger claims no originality of his question below.
Find the equivalent resistance, in terms of 
, 
, 
, and 
, when the switch is
i. open;
ii. closed.
Solution.
i. When the switch is open, we draw the following diagram:
The equivalent resistance of and in series is their sum 
. Similarly, that of and in series is 
. The equivalent resistance 
of and in parallel is given by 
. Thus 
.
ii. When the switch is closed, we draw the following diagram:
The equivalent resistance of and in parallel is 
. Similarly, that of and in parallel is 
. Thus the equivalent resistance of them in series is 
.