# Thread: Double and Half Angles

1. ## Double and Half Angles

What is the purpose of these formulas?
Why do we use them and what do they tell us?

For example if CosA=5/9 what is Cos2A? what does 31/81 tell me?

2. Originally Posted by algebraisabeast
What is the purpose of these formulas?
Why do we use them and what do they tell us?

For example if CosA=5/9 what is Cos2A? what does 31/81 tell me?
I'd use the double angle formula in cos: $cos(2\theta) = 2cos^2\theta - 1$

3. I know the formula that I have to use. I know that since i have a value I can get the angle if I place that value in I can get the double angle. What is the point of the double angle? Why would I even look for this? Why do I need to be taught this?

4. Originally Posted by algebraisabeast
I know the formula that I have to use. I know that since i have a value I can get the angle if I place that value in I can get the double angle. What is the point of the double angle? Why would I even look for this? Why do I need to be taught this?
HI

if you were to find cos 60 without using the calculator , how would you do this ?

Knowing the double angles would help a lot .

cos 60 = cos 2(30)

=2 cos^2 (30) -1

$=2(\frac{\sqrt{3}}{2})^2-1$ and go on .

Of course this is only one of its usage .