solve the CIRCUIT..............

we cant consider becoz same current is not passing through them

(4,1) cant be taken in series becoz in between them there is a capacitor C2.

see,, for capacitors to be in series, the charge in each capacitor is Q.

consider the junction between 4 and 1;
can you say .. charge in 4 and 1 are same?

you cant; becoz:- there will be some charge going to the lower plate of C2 as well..

ne more doubt ?

In

the wire in red shows that pts. 'a' and 'b' are at the same potential.So the can be joined ; and the the circuit becomes

and this becomes very simple.