"Principal of Duality"

According to principle of duality "Dual of one expression is

obtained by replacing AND (.)with OR(+) and OR with AND together with replacement of 1 with 0 and

0 with 1.

For example: consider the expression A+B=0. The dual of this expression is obtained by replacing + with . and 0 by 1.

i.e., A.B=1 is dual of A+B=0