Dont Cares

These don't care conditions are like bonus points in any game. Depending on the requirement these don't cares can be used as 1's or 0's. For example, You can assume them as 1's to construct a big group so that the logic can be as minimum as possible. But, if some of them are not useful for grouping, you can simply ignore them as if they are '0'. It is always a win-win situation when you know don't cares of your design, these can significantly reduce lot of logic.

dont_cares_karnaugh_group

In the above map, you can see that how easily few don't cares are gathered to make a big group.

So the logic for this group will be

D.B    

And the logic for isolated one will is..

  - - -
A.B.C.D    

So the final boolean expression will become..

        - - - 
D.B + A.B.C.D 

That's the beauty of "don't cares", when you need them they can be 1's for you, and when you don't, they will be simply 0's.