An example of an open sentence: x + 3 = 6
First ... what is a "Sentence" ?
Just like an English sentence, in mathematics a sentence says something:
Now ... what is a "Closed Sentence" or an "Open Sentence" ?
|A closed sentence is always true (or always false).|
|A sentence is open when it is not known if it is true or false.|
|8 is an even number||is closed (it is always true)|
|9 is an even number||is closed (it is always false)|
|n is an even number||is open (could be true or false, depending on the value of n)|
In that last example:
- if n was 4 the sentence would be true,
- if n was 5 the sentence would be false,
- etc ...
But we didn't say what value n has!
So "n is an even number" may be true or false. It is therefore open.
So, we get this definition:
An open sentence can be either true or false depending what values are used.
The value we don't know is called a variable (also called an unknown)
In this example of an open sentence, x is a variable:
x + 3 = 8
In this example, w and q are both variables:
w + q = 2
Solving means finding a value for the variable that makes the sentence true.
Example: Solve x + 3 = 8
Let us subtract 3 from both sides:
x + 3 - 3 = 8 - 3
x = 5
Check: 5 + 3 = 8 is true
So we have solved x + 3 = 8 by making x = 5
Some More Examples
Here are some more examples of closed and open sentences for you:
|A square has four corners||always true|
|6 is less than 5||always false|
|-3 is a negative number||always true|
|A triangle has n sides||Can be true or false (depending on the value of n)|
|z is a positive number||Can be true or false (depending on the value of z)|
|3y = 4x + 2||Can be true or false (depending on the values of x and y)|
|a + b = c + d||Can be true or false (depending on the values of a,b,c,d)|