# 4th derivative of cos(2x)

see title.

no

## The Attempt at a Solution

Ok so the solution is 16cos(2x) but I'm not sure how it is derived to that. I've tried the product rule but it's not working for me. What rule or rules do I use to get this solution?

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Well if you calculate the first derivative properly you should be yielded to -2sin(2x).

Edit: Take into consideration that the derivative of cos(Ux)= -u'(x)*sin(x)

Ok cool but what rule did you use there?

The chain rule.

HallsofIvy
Homework Helper
You don't need the product rule because do not have a product of two functions of x. You need the chain rule because you have f(y)= 2cos(y) and y= 2x:
$$\frac{df}{dx}= \frac{df}{dy}\frac{dy}{dx}$$
With f(y)= cos(y), what is df/dy? With y= 2x, what is dy/dx?

Mark44
Mentor
You don't need the product rule because do not have a product of two functions of x. You need the chain rule because you have f(y)= 2cos(y) and y= 2x:
Make that f(y)= cos(y)
$$\frac{df}{dx}= \frac{df}{dy}\frac{dy}{dx}$$
With f(y)= cos(y), what is df/dy? With y= 2x, what is dy/dx?

HallsofIvy