Law of Tangents

[Guest]

I found the law of tangents online but I have never seen it in ANY textbook. Why?

Here it is:

The Law of Tangents : (a-b)/(a+b) = tg (a/2 - b/2)/ tg (a/2 + b/2)

What math course will teach this tangent law?

 

[tkhunny]

Some things are for special interest or historical reference only.

What does the "Law" do that cannot be done in some simpler way?