Originally Posted by **evenkeel3x**

which Double angle formula do you use for cos(2x) ? I've tried all three

Use the identity for cos(2x) that allows you to make cancellations,

arriving at sin(x)/cos(x).

I mean, you have three factors in the denominator, yes?

2 * sin(x) * cos(x)

You want only cos(x) in the denominator, yes?

This means that you need to find some way to cancel both the factor of 2

and the factor of sin(x).

So, when you're looking at the possibilities for cos(2x), be thinking:

"I need a factor of 2 on top (to cancel), and

I need a factor of sin(x) on top (to cancel)".