This textbook is a 'new mathematics textbook' written by the author through a synthesis of his past research and education, providing a new understanding of mathematics from the basics.
Unlike ordinary mathematics books, this book is much more free-flowing in its content, and if you follow the author's thoughts that mathematics is a flow, not a trick, you will naturally encounter a clear flow called mathematics.

The addition theorem for sines and cosines is one of the most important formulas that will appear in this lecture.
I expect you to remember these formulas well.
First, I would like you to read these formulas out loud several times here and there.
(Of course, this type of formula has the advantage of being memorized automatically over time.
Anyway, these formulas are very important.
I emphasize this.) Using the addition theorem, we can do, for example, the following calculations...

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