The order is the content of compulsory five. This textbook includes three chapters, namely "Solving Triangle", "Sequence" and "Inequality". This book requires students to correctly use mathematical knowledge to solve practical problems in life.

Sequence is a function with positive integer set (or its finite set) as its domain, and it is an ordered number. Every number in a series is called an item in this series. The number ranked first is called 1 item of this series (usually also called the first item), the number ranked second is called the second item of this series, and so on, and the number ranked n is called the n item of this series, usually represented by an.