# Homework Help: Truth tables - I don't get it

1. Feb 10, 2010

### XodoX

1. Demonstrate by means of truth tables the validity of the following identities.

a) DeMorgan's theorem for two variables

X'Y' = X' + Y'

' is the bars that are above the letters.

Can someone explain this to me? I mean the truth table is just gonna look like this: X'Y' X' Y'
I just need to know where and 0's and 1's go and why. Seems so easy, but I haven't been able to understand it.

2. Feb 10, 2010

### Staff: Mentor

Your table will have 4 coulmns. Fist is X, second is Y, third is X'Y' and forth is X' + Y'.

Well, except that I think you forgot something in your statement of DeMorgan's Law.

EDIT -- it's just a typo in how you wrote it out...

3. Feb 10, 2010

### XodoX

Thanks. I just thought it was 3. How do I find out about the 0's and 1's ? I know DeMorgan's but don't know how to apply it to this one.

4. Feb 10, 2010

### Staff: Mentor

(hint -- check out the wikipedia.org page on DeMorgan's Law....)