Step by step
The question
Question: are p ∨ (q ∧ r) and (p ∨ q) ∧ (p ∨ r) equivalent?
1 Rows for p, q and r
Each letter can be true or false.
Three letters give 2 × 2 × 2 = 8 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: two T, two F, and again.
r: T, F, T, F, down the column.
2 Column for q ∧ r
We read this sign as and.
q ∧ r is true only when q and r are both true.
So it is T only in rows 1 and 5.
3 Column for p ∨ (q ∧ r)
We read this sign as or.
p ∨ (q ∧ r) is true when at least one of them is true.
It is false only when both are false.
So it is F only in rows 6, 7 and 8.
4 Column for p ∨ q
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 rows 7 and 8.
5 Column for p ∨ r
p ∨ r is true when at least one of them is true.
It is false only when both are false.
So it is F only in rows 6 and 8.
6 Column for (p ∨ q) ∧ (p ∨ r)
(p ∨ q) ∧ (p ∨ r) is true only when p ∨ q and p ∨ r are both true.
So it is F only in rows 6, 7 and 8.
7 Compare the columns
Now compare the two last columns.
p ∨ (q ∧ r) in red, (p ∨ q) ∧ (p ∨ r) in purple.
Row by row they are the same: T, T, T, T, T, F, F, F and T, T, T, T, T, F, F, F.
This is the distributive law, like opening brackets.
Careful!
To be equivalent they must match in every row.
One row that differs is enough to say no.
So they are equivalent.
We write p ∨ (q ∧ r) ≡ (p ∨ q) ∧ (p ∨ r).
8 Check
Let's check row 2 by hand.
In row 2, p is true, q is true and r is false.
The first: T ∨ (T ∧ F) gives T.
And the second: (T ∨ T) ∧ (T ∨ F) gives T.
The same, as the table says.
Right!
Let's look back.
First: rows for p, q and r.
Then: column for q ∧ r.
Then: column for p ∨ (q ∧ r).
Then: column for p ∨ q.
Then: column for p ∨ r.
Then: column for (p ∨ q) ∧ (p ∨ r).
Then: compare the columns.
Last: check.