Like other branches of science, trigonometry stemmed from social needs. The development of the maritime industry required sailors to locate position of their ships in the ocean by the sun by day and by stars by night. Wars required determining long distances and making maps. Farmers needed to find out about weather changes during a year to arrange their production in time for their crops, so calendars appeared and so on.
The needs above made it possible for trigonometry to appear and develop. First of all, Greek mathematicians made great contributions to the development of trigonometry and then Euler developed Modern Theory on Trigonometric Functions in his book "Introduction to Analysis of the Infinite" published in 1748.
Dear friends, this book presents the most basic knowledge of trigonometry.
If you are a student, the book is a very useful reference for you to study and test.
At the end of each chapter there are exercises that have the solution, before you read the solution, you should solve the exercises yourself first.
Wish you study and learn well