Are ~(p or q) and ~p and ~q equivalent

Step by step

The question

Question: are ¬(p ∨ q) and ¬p ∧ ¬q equivalent?

1 Rows for p and q

Each letter can be true or false.

Two letters give 2 × 2 = 4 rows: every way they can go.

We make a table: a column for each letter, then a column for each part, from the inside out.

We write T for true and F for false.

p: the first half T, the second half F.

This way no row is missed.

q: T, F, T, F, down the column.

2 Column for p ∨ q

We read this sign as or.

p ∨ q is true when at least one of them is true.

It is false only when both are false.

So it is F only in row 4.

3 Column for ¬(p ∨ q)

We read this sign as not.

¬(p ∨ q) is the opposite of p ∨ q: T becomes F, F becomes T.

So it is T only in row 4.

4 Column for ¬p and ¬q

¬p is the opposite of p: T becomes F, F becomes T.

So it is T only in rows 3 and 4.

¬q is the opposite of q too.

5 Column for ¬p ∧ ¬q

We read this sign as and.

¬p ∧ ¬q is true only when ¬p and ¬q are both true.

So it is T only in row 4.

6 Compare the columns

Now compare the two last columns.

¬(p ∨ q) in red, ¬p ∧ ¬q in purple.

Row by row they are the same: F, F, F, T and F, F, F, T.

This is De Morgan's law: the not goes onto each part, and the sign in the middle swaps.

Careful!

The not goes onto each part, and the or turns into and.

So they are equivalent.

We write ¬(p ∨ q) ≡ ¬p ∧ ¬q.

7 Check

Let's check row 2 by hand.

In row 2, p is true and q is false.

The first: ¬(T ∨ F) gives F.

And the second: ¬T ∧ ¬F gives F.

The same, as the table says.

Right!

Let's look back.

First: rows for p and q.

Then: column for p ∨ q.

Then: column for ¬(p ∨ q).

Then: column for ¬p and ¬q.

Then: column for ¬p ∧ ¬q.

Then: compare the columns.

Last: check.