You learned in Math 9 and Math 10 that expressions with variables allow you to replace the variable with a number, and evaluate the result.

'Evaluate' means fill in the given number and work it out.

For example, evaluate x2 + 3x + 1 when x is 4

x2 + 3x + 1

= (4)2 + 3(4) + 1

= 29

However, there are some circumstances when you cannot work out the answer.
Here's an example of one of those:

Evaluate when x = -3

When you fill in -3 for x, the bottom of this expression becomes zero, since -3 + 3 = 0

For this expression, -3 is called an undefined value, or a restriction, or a non-permissible value.
It's the value of x that would make the bottom of a fraction equal to zero ... and division by zero is impossible.

So when you're asked to find the non-permissible values of an expression, you need to find the values of x that would make the denominator work out to zero. Those are the values that can't be used.

Let's go to page two and look at some more examples ...

Mr. Willis' Page