A number in the complex plane. It can be written as , where and are real numbers and .

A function is a rule that takes inputs from one set and assigns them to outputs from another set. Each input is assigned exactly one output.

A number on the imaginary number line. It can be written as , where is a real number and .

An irrational number is a number that is not rational. This means it cannot be expressed as a positive fraction, a negative fraction, or zero. It cannot be written in the form , where and are integers and .

For example, the numbers and are irrational numbers.

A rational number is a number that can be written as a positive fraction, a negative fraction, or zero. It can be written in the form , where and are integers and . 

• 0.7 is a rational number because it can be written as .
• The numbers 0, 3, , and 6.7 are all rational numbers. 

A number on the real number line.