1. The problem statement, all variables and given/known data [PLAIN]http://img340.imageshack.us/img340/7690/123rk.gif [Broken] 2. Relevant equations The answer should be A'C' + A'D' 3. The attempt at a solution Shown in above in the question. I want to know how can i get the correct answer. As you see above, in my last step, I wonder whether B'+BD and B'+BC are equal to 1 ? If so, then I can get the answer. But if dont, where is my problem? Thanks.