##### Michael Schoch, Noodle Intern

Keep in mind that any number can be plugged into the "x", or variable, in a rational expression. If a question asks you why the variable cannot be a particular number it is really asking you why that number will cause the expression to not be rational.

Here is a video that explains rational expressions. Additionally, here is a sample problem that involves simplifying a rational expression.

It's maybe most helpful to remember that an expression cannot be rational if it involves a number being divided by zero.

I hope that helps!