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# 2.6: Truth Tables

Difficulty Level: At Grade Created by: CK-12

What if you needed to analyze a complex logical argument? How could you do this is an organized way, making sure to account for everything? After completing this Concept, you'll be able to use truth tables as a way to organize and analyze logic.

### Guidance

So far we know these symbols for logic:

• \begin{align*}\sim\end{align*} not (negation)
• \begin{align*}\rightarrow\end{align*} if-then
• \begin{align*}\therefore\end{align*} therefore

Two more symbols are:

• \begin{align*}\land\end{align*} and
• \begin{align*}\lor\end{align*} or

We would write “p\begin{align*}p\end{align*} and q\begin{align*}q\end{align*}” as pq\begin{align*}p \land q\end{align*} and “p\begin{align*}p\end{align*} or q\begin{align*}q\end{align*}” as pq\begin{align*}p \lor q\end{align*}.

Truth tables use these symbols and are another way to analyze logic. First, let’s relate p\begin{align*}p\end{align*} and p\begin{align*}\sim p\end{align*}. To make it easier, set p\begin{align*}p\end{align*} as: An even number. Therefore, p\begin{align*}\sim p\end{align*} is An odd number. Make a truth table to find out if they are both true. Begin with all the “truths” of p\begin{align*}p\end{align*}, true (T) or false (F).

p\begin{align*}p\end{align*}
T\begin{align*}T\end{align*}
F\begin{align*}F\end{align*}

Next we write the corresponding truth values for p\begin{align*}\sim p\end{align*}. p\begin{align*}\sim p\end{align*} has the opposite truth values of p\begin{align*}p\end{align*}. So, if p\begin{align*}p\end{align*} is true, then p\begin{align*}\sim p\end{align*} is false and vise versa.

p\begin{align*}p\end{align*} p\begin{align*}\sim p\end{align*}
T F
F T

To Recap:

• Start truth tables with all the possible combinations of truths. For 2 variables there are 4 combinations for 3 variables there are 8. You always start a truth table this way.
• Do any negations on the any of the variables.
• Do any combinations in parenthesis.
• Finish with completing what the problem was asking for.

#### Example A

Draw a truth table for p,q\begin{align*}p, q\end{align*} and pq\begin{align*}p \land q\end{align*}.

First, make columns for p\begin{align*}p\end{align*} and q\begin{align*}q\end{align*}. Fill the columns with all the possible true and false combinations for the two.

p\begin{align*}p\end{align*} q\begin{align*}q\end{align*}
T\begin{align*}T\end{align*} T\begin{align*}T\end{align*}
T\begin{align*}T\end{align*} F\begin{align*}F\end{align*}
F\begin{align*}F\end{align*} T\begin{align*}T\end{align*}
F\begin{align*}F\end{align*} F\begin{align*}F\end{align*}

Notice all the combinations of p\begin{align*}p\end{align*} and q\begin{align*}q\end{align*}. Anytime we have truth tables with two variables, this is always how we fill out the first two columns.

Next, we need to figure out when pq\begin{align*}p \land q\end{align*} is true, based upon the first two columns. pq\begin{align*}p \land q\end{align*} can only be true if BOTH p\begin{align*}p\end{align*} and q\begin{align*}q\end{align*} are true. So, the completed table looks like this:

This is how a truth table with two variables and their “and” column is always filled out.

#### Example B

Draw a truth table for p,q\begin{align*}p, q\end{align*} and pq\begin{align*}p \lor q\end{align*}.

First, make columns for p\begin{align*}p\end{align*} and q\begin{align*}q\end{align*}, just like Example A.

p\begin{align*}p\end{align*} q\begin{align*}q\end{align*}
T\begin{align*}T\end{align*} T\begin{align*}T\end{align*}
T\begin{align*}T\end{align*} F\begin{align*}F\end{align*}
F\begin{align*}F\end{align*} T\begin{align*}T\end{align*}
F\begin{align*}F\end{align*} F\begin{align*}F\end{align*}

Next, we need to figure out when pq\begin{align*}p \lor q\end{align*} is true, based upon the first two columns. pq\begin{align*}p \lor q\end{align*} is true if p\begin{align*}p\end{align*} OR q\begin{align*}q\end{align*} are true, or both are true. So, the completed table looks like this:

The difference between pq\begin{align*}p \land q\end{align*} and pq\begin{align*}p \lor q\end{align*} is the second and third rows. For “and” both p\begin{align*}p\end{align*} and q\begin{align*}q\end{align*} have to be true, but for “or” only one has to be true.

#### Example C

Determine the truths for p(qr)\begin{align*}p \land (\sim q \lor r)\end{align*}.

First, there are three variables, so we are going to need all the combinations of their truths. For three variables, there are always 8 possible combinations.

p\begin{align*}p\end{align*} q\begin{align*}q\end{align*} r\begin{align*}r\end{align*}
T\begin{align*}T\end{align*} T\begin{align*}T\end{align*} T\begin{align*}T\end{align*}
T\begin{align*}T\end{align*} T\begin{align*}T\end{align*} F\begin{align*}F\end{align*}
T\begin{align*}T\end{align*} F\begin{align*}F\end{align*} T\begin{align*}T\end{align*}
T\begin{align*}T\end{align*} F\begin{align*}F\end{align*} F\begin{align*}F\end{align*}
F\begin{align*}F\end{align*} T\begin{align*}T\end{align*} T\begin{align*}T\end{align*}
F\begin{align*}F\end{align*} T\begin{align*}T\end{align*} F\begin{align*}F\end{align*}
F\begin{align*}F\end{align*} F\begin{align*}F\end{align*} T\begin{align*}T\end{align*}
F\begin{align*}F\end{align*} F\begin{align*}F\end{align*} F\begin{align*}F\end{align*}

Next, address the q\begin{align*}\sim q\end{align*}. It will just be the opposites of the q\begin{align*}q\end{align*} column.

p\begin{align*}p\end{align*} q\begin{align*}q\end{align*} r\begin{align*}r\end{align*} q\begin{align*}\sim q\end{align*}
T\begin{align*}T\end{align*} T\begin{align*}T\end{align*} T\begin{align*}T\end{align*} F\begin{align*}F\end{align*}
T\begin{align*}T\end{align*} T\begin{align*}T\end{align*} F\begin{align*}F\end{align*} F\begin{align*}F\end{align*}
T\begin{align*}T\end{align*} F\begin{align*}F\end{align*} T\begin{align*}T\end{align*} T\begin{align*}T\end{align*}
T\begin{align*}T\end{align*} F\begin{align*}F\end{align*} F\begin{align*}F\end{align*} T\begin{align*}T\end{align*}
F\begin{align*}F\end{align*} T\begin{align*}T\end{align*} T\begin{align*}T\end{align*} F\begin{align*}F\end{align*}
F\begin{align*}F\end{align*} T\begin{align*}T\end{align*} F\begin{align*}F\end{align*} F\begin{align*}F\end{align*}
F\begin{align*}F\end{align*} F\begin{align*}F\end{align*} T\begin{align*}T\end{align*} T\begin{align*}T\end{align*}
F\begin{align*}F\end{align*} F\begin{align*}F\end{align*} F\begin{align*}F\end{align*} T\begin{align*}T\end{align*}

Now, let’s do what’s in the parenthesis, qr\begin{align*}\sim q \lor r\end{align*}. Remember, for “or” only q\begin{align*}\sim q\end{align*} OR r\begin{align*}r\end{align*} has to be true. Only use the q\begin{align*}\sim q\end{align*} and r\begin{align*}r\end{align*} columns to determine the values in this column.

p\begin{align*}p\end{align*} q\begin{align*}q\end{align*} r\begin{align*}r\end{align*} q\begin{align*}\sim q\end{align*} qr\begin{align*}\sim q \lor r\end{align*}
T\begin{align*}T\end{align*} T\begin{align*}T\end{align*} T\begin{align*}T\end{align*} F\begin{align*}F\end{align*} T\begin{align*}T\end{align*}
T\begin{align*}T\end{align*} T\begin{align*}T\end{align*} F\begin{align*}F\end{align*} F\begin{align*}F\end{align*} F\begin{align*}F\end{align*}
T\begin{align*}T\end{align*} F\begin{align*}F\end{align*} T\begin{align*}T\end{align*} T\begin{align*}T\end{align*} T\begin{align*}T\end{align*}
T\begin{align*}T\end{align*} F\begin{align*}F\end{align*} F\begin{align*}F\end{align*} T\begin{align*}T\end{align*} T\begin{align*}T\end{align*}
F\begin{align*}F\end{align*} T\begin{align*}T\end{align*} \begin{align*}T\end{align*} \begin{align*}F\end{align*} \begin{align*}T\end{align*}
\begin{align*}F\end{align*} \begin{align*}T\end{align*} \begin{align*}F\end{align*} \begin{align*}F\end{align*} \begin{align*}F\end{align*}
\begin{align*}F\end{align*} \begin{align*}F\end{align*} \begin{align*}T\end{align*} \begin{align*}T\end{align*} \begin{align*}T\end{align*}
\begin{align*}F\end{align*} \begin{align*}F\end{align*} \begin{align*}F\end{align*} \begin{align*}T\end{align*} \begin{align*}T\end{align*}

Finally, we can address the entire problem, \begin{align*}p \land (\sim q \lor r)\end{align*}. Use the \begin{align*}p\end{align*} and \begin{align*}\sim q \lor r\end{align*} to determine the values. Remember, for “and” both \begin{align*}p\end{align*} and \begin{align*}\sim q \lor r\end{align*} must be true.

\begin{align*}p\end{align*} \begin{align*}q\end{align*} \begin{align*}r\end{align*} \begin{align*}\sim q\end{align*} \begin{align*}\sim q \lor r\end{align*} \begin{align*}p \land(\sim q \lor r)\end{align*}
\begin{align*}T\end{align*} \begin{align*}T\end{align*} \begin{align*}T\end{align*} \begin{align*}F\end{align*} \begin{align*}T\end{align*} \begin{align*}T\end{align*}
\begin{align*}T\end{align*} \begin{align*}T\end{align*} \begin{align*}F\end{align*} \begin{align*}F\end{align*} \begin{align*}F\end{align*} \begin{align*}F\end{align*}
\begin{align*}T\end{align*} \begin{align*}F\end{align*} \begin{align*}T\end{align*} \begin{align*}T\end{align*} \begin{align*}T\end{align*} \begin{align*}T\end{align*}
\begin{align*}T\end{align*} \begin{align*}F\end{align*} \begin{align*}F\end{align*} \begin{align*}T\end{align*} \begin{align*}T\end{align*} \begin{align*}T\end{align*}
\begin{align*}F\end{align*} \begin{align*}T\end{align*} \begin{align*}T\end{align*} \begin{align*}F\end{align*} \begin{align*}T\end{align*} \begin{align*}F\end{align*}
\begin{align*}F\end{align*} \begin{align*}T\end{align*} \begin{align*}F\end{align*} \begin{align*}F\end{align*} \begin{align*}F\end{align*} \begin{align*}F\end{align*}
\begin{align*}F\end{align*} \begin{align*}F\end{align*} \begin{align*}T\end{align*} \begin{align*}T\end{align*} \begin{align*}T\end{align*} \begin{align*}F\end{align*}
\begin{align*}F\end{align*} \begin{align*}F\end{align*} \begin{align*}F\end{align*} \begin{align*}T\end{align*} \begin{align*}T\end{align*} \begin{align*}F\end{align*}

Watch this video for help with the Examples above.

### Vocabulary

Truth tables use symbols to analyze logic.

### Guided Practice

Write a truth table for the following variables.

1. \begin{align*}p \land \sim p\end{align*}

2. \begin{align*}\sim p \lor \sim q\end{align*}

3. \begin{align*}p \land (q \lor \sim q)\end{align*}

1. First, make columns for \begin{align*}p\end{align*}, then add in \begin{align*}\sim p\end{align*} and finally, evaluate \begin{align*} p \land \sim p\end{align*}.

\begin{align*}p\end{align*} \begin{align*}\sim p\end{align*} \begin{align*}p\land \sim p\end{align*}
\begin{align*}T\end{align*} \begin{align*}F\end{align*} \begin{align*}F\end{align*}
\begin{align*}F\end{align*} \begin{align*}T\end{align*} \begin{align*}F\end{align*}

2. First, make columns for \begin{align*}p\end{align*} and \begin{align*}q\end{align*}, then add in \begin{align*}\sim p\end{align*} and \begin{align*}\sim q\end{align*}. Finally, evaluate \begin{align*}\sim p \lor \sim q\end{align*}.

\begin{align*}p\end{align*} \begin{align*}q\end{align*} \begin{align*}\sim p\end{align*} \begin{align*}\sim q\end{align*} \begin{align*}\sim p \lor \sim q\end{align*}
\begin{align*}T\end{align*} \begin{align*}T\end{align*} \begin{align*}F\end{align*} \begin{align*}F\end{align*} \begin{align*}F\end{align*}
\begin{align*}T\end{align*} \begin{align*}F\end{align*} \begin{align*}F\end{align*} \begin{align*}T\end{align*} \begin{align*}T\end{align*}
\begin{align*}F\end{align*} \begin{align*}T\end{align*} \begin{align*}T\end{align*} \begin{align*}F\end{align*} \begin{align*}T\end{align*}
\begin{align*}F\end{align*} \begin{align*}F\end{align*} \begin{align*}T\end{align*} \begin{align*}T\end{align*} \begin{align*}T\end{align*}

3. First, make columns for \begin{align*}p\end{align*} and \begin{align*}q\end{align*}, then add in \begin{align*}\sim q\end{align*} and \begin{align*}q \lor \sim q\end{align*}. Finally, evaluate \begin{align*}p \land (q \lor \sim q)\end{align*}.

\begin{align*}p\end{align*} \begin{align*}q\end{align*} \begin{align*}\sim q\end{align*} \begin{align*}(q \lor \sim q)\end{align*} \begin{align*}p \land (q \lor \sim q)\end{align*}
\begin{align*}T\end{align*} \begin{align*}T\end{align*} \begin{align*}F\end{align*} \begin{align*}T\end{align*} \begin{align*}T\end{align*}
\begin{align*}T\end{align*} \begin{align*}F\end{align*} \begin{align*}T\end{align*} \begin{align*}T\end{align*} \begin{align*}T\end{align*}
\begin{align*}F\end{align*} \begin{align*}T\end{align*} \begin{align*}F\end{align*} \begin{align*}T\end{align*} \begin{align*}F\end{align*}
\begin{align*}F\end{align*} \begin{align*}F\end{align*} \begin{align*}T\end{align*} \begin{align*}T\end{align*} \begin{align*}F\end{align*}

### Practice

Write a truth table for the following variables.

1. \begin{align*}(p \land q) \lor \sim r\end{align*}
2. \begin{align*}p \lor (\sim q \lor r)\end{align*}
3. \begin{align*}p \land (q \lor \sim r)\end{align*}
4. The only difference between #1 and #3 is the placement of the parenthesis. How do the truth tables differ?
5. When is \begin{align*}p \lor q \lor r\end{align*} true?
6. \begin{align*}p \lor q \lor r\end{align*}
7. \begin{align*}(p \lor q) \lor \sim r\end{align*}
8. \begin{align*}(\sim p \land \sim q) \land r\end{align*}
9. \begin{align*}(\sim p \lor \sim q) \land r\end{align*}

Is the following a valid argument? If so, what law is being used? HINT: Statements could be out of order.

Jul 17, 2012

Aug 24, 2015

# We need you!

At the moment, we do not have exercises for Truth Tables.