There are two solutions in one image-charge problem?

[garylau]

Sorry
there are two different solutions in one question
! which one is correct??
the first one is written by griffiths

the second one is written by a lecturer.

they are not consistent in the same question
So which one is correct?
thank you

 

[BvU]

Well, in the preceding section Griffiths gives an alternative way to calculate the work !
It's not too complicated to integrate the force you calculated over the path bx = ay from $\infty$ to $(a,b)$ .
(And you can simplify by setting $a=b$ -- all you're after is factor of 2).

This thread should be in the homework forum.

 

[berkeman]

This thread should be in the homework forum.

It looks more like a studying question so far, so we can leave it here for now.