Find electric potential at center `O`.
(a)

Explanation

(a) `V_(0) = (1)/(4 pi in_(0)).(Q)/(R ) xx 3 , r = (d)/(sqrt(3))`
`= (1)/(4 pi in_(0)) . (3 sqrt(3) Q)/(d)`
(b) `V_(0) = (1)/( 4 pi in_(0))[(Q)/(r ) + (2Q)/(r ) + (3 Q)/(r ) + (4 Q)/(r )] , r = d`
`= (1)/( 4 pi in_(0)). (10 Q)/(r )`
`= (1)/(4 pi in_(0)) (10 Q)/(d)`