# Thread: Trigonometric Derivative using Identity

1. ## Trigonometric Derivative using Identity

Use the identity cos 2x = cosx * cos x - sin x * sin x to find the derivative of cos 2x. Express the derivative in terms of sin 2x.

I have no idea how to do this.

2. Originally Posted by Truthbetold
Use the identity cos 2x = cosx * cos x - sin x * sin x to find the derivative of cos 2x. Express the derivative in terms of sin 2x.

I have no idea how to do this.
just let $\displaystyle y = \cos x \cos x - \sin x \sin x$ and use the product rule

3. Originally Posted by Jhevon
just let $\displaystyle y = \cos x \cos x - \sin x \sin x$ and use the product rule
Product rule of both sides?

Get: -cos sin + cos sin - (-cos sin - cos sin)
And get: 0

That cannot be the right answer.

What am I doing wrong?

4. Originally Posted by Truthbetold
Product rule of both sides?

Get: -cos sin + cos sin - (-cos sin - cos sin)
And get: 0

That cannot be the right answer.

What am I doing wrong?
the derivative of sin(x) is +cos(x)

and it's a bad habit to write sin and cos alone like that.