#1**+1 **

What rule is this? What is it called? Sin(2nX)/sin(nX) = 2Cos(nX)

Hello guest!

sin 2x = 2 sin x cos x is one of the double angle functions

sin(2X)=2sin(X)*cos(X)

One can show this nicely over an** addition theorem:**

**sin(x+y) = sin(x) * cos(y) + sin(y) * cos(x)**

and this case has to do with double angles ...

sin(2x) = sin(x+x) = sin(x) * cos(x) + sin(x) * cos(x) = 2*sin(x)*cos(x)

!

asinus
Aug 9, 2017

#2**+1 **

If I don't mind asking. Other than addition theorm what other rules do I need to learn, that is in the same level of learning, concerning sin and cos.

I'm abit behind in mathematics. I think half-angle trigonetric formulae is one of them.

Guest Aug 9, 2017

#3**+1 **

Dear learner of trigonometry!

http://mathworld.wolfram.com/TrigonometricAdditionFormulas.html

Study this, please!

I wish you great success.

!

asinus
Aug 9, 2017