‘IF ... THEN ...’ SENTENCES

One of the most common sentence structures in mathematics is ‘If $\,A\,$, then $\,B\,$’.
This type of sentence is used in English, too:   for example, ‘If it's raining, then the ground is wet.’
However, this sentence type is much more important in mathematics.

Sentences of the form ‘If [beautiful math coming... please be patient] $\,A\,$, then $\,B\,$’ are called conditional sentences or implications.

Because this sentence type is so important, there are many different ways to say the same thing, as follows:

The following are equivalent:   that is,
if one sentence is true, then every sentence is true;
and if one sentence is false, then every sentence is false.

In all these sentences, [beautiful math coming... please be patient] $\,A\,$ is called the hypothesis and $\,B\,$ is called the conclusion.

If [beautiful math coming... please be patient] $\,A\,$, then $\,B\,$ Be sure that every if  has a then!
$\,B\,$, if $\,A\,$ Some people state the conclusion first, to give it emphasis.
$\,A\,$ implies $\,B\,$
$\,A\Rightarrow B\,$ read this as ‘$\,A\,$ implies $\,B\,$’
Whenever $\,A\,$, $\,B\,$ Some people prefer the word whenever to the word if .
If you use the word whenever then it is conventional to leave out the word then.
$\,B\,$, whenever $\,A\,$ Some people state the conclusion first, to give it emphasis.
$\,A\,$ is sufficient for $\,B\,$

You will see in the next section that ‘If [beautiful math coming... please be patient] $\,A\,$, then $\,B\,$’ is not equivalent to ‘If $\,B\,$, then $\,A\,$’.
Therefore, the positions of $\,A\,$ and $\,B\,$ in these sentences is important. Be careful about this.

The sentence ‘If [beautiful math coming... please be patient] $\,A\,$, then $\,B\,$’ is a compound sentence:
$\,A\,$ is a sentence, which can be true or false;
$\,B\,$ is a sentence, which can be true or false;
the truth of the compound sentence ‘If $\,A\,$, then $\,B\,$’ depends on the truth of its subsentences $\,A\,$ and $\,B\,$.

To define a compound sentence, we must state its truth (true or false) for all possible combinations of its subsentences,
and this is done by using a truth table:

DEFINITION implication
hypothesis
$\,A\,$
conclusion
$\,B\,$
implication
If $\,A\,$, then $\,B\,$
TTT
TFF
FTT
FFT

The rows of the truth table are always written in the order given in this table.

Here are some important observations from the truth table:

Lines 3 and 4 are usually hardest for beginning students of logic to understand, so I like to use this analogy:

Suppose your parents have said to you, ‘If you get a [beautiful math coming... please be patient] $\,90\,$ or above in AP Calculus, then we'll buy you a car.’
Now, suppose they are telling the truth (that is, suppose the implication is true).
If you get a $\,90\,$ or above, then they must buy you a car. (line 1 of the truth table)
Suppose, however, that you earn a grade less than $\,90\,$. (lines 3 or 4)
They could still choose to buy you a car, since they know how hard you worked. (line 3)
Or, they could put this money towards college instead, and not buy the car. (line 4)

To prove that a given implication is always true, you need to verify that line 2 of the truth table can never occur.
Thus, you want to show that whenever the hypothesis is true, the conclusion must also be true.

This approach is called a direct proof of the implication:

(There are other types of proofs, which will be discussed in future sections.)

Master the ideas from this section
by practicing the exercise at the bottom of this page.

When you're done practicing, move on to:
Contrapositive and Converse


On this exercise, you will not key in your answer.
However, you can check to see if your answer is correct.
PROBLEM TYPES:
1 2 3 4 5 6 7 8 9 10
11 12 13 14 15 16 17 18 19  
AVAILABLE MASTERED IN PROGRESS

(MAX is 19; there are 19 different problem types.)