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# Differentiation

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1. 1. d/dx[(dy/dz)(cosx)]

2. As x,y,z->0, (delta(x)delta(y))/delta(z) = 0 right? If only y and z -> 0, = delta(x)dy/dz?
2. Could you post a screenshot of the question itself.
3. (Original post by Vikingninja)
Could you post a screenshot of the question itself.
That's my question.
4. (Original post by AlmostNotable)
1. d/dx[(dy/dz)(cosx)]
Use the product rule.

5. (Original post by notnek)
Use the product rule.

(d2y/dx2)(dy/dx)(cosx)-(dy/dx)(sinx), where is the mistake?

Original question : As x,y,z->0, (delta(x)delta(y))/delta(z) = 0 right? If only y and z -> 0, = delta(x)dy/dz?
6. (Original post by AlmostNotable)
(d2y/dx2)(dy/dx)(cosx)-(dy/dx)(sinx), where is the mistake?
It's nearly right but where did the 'z' go?
7. (Original post by notnek)
It's nearly right but where did the 'z' go?
(d2y/dz2)(dy/dx)(cosx)-(dy/dz)(sinx), where is the mistake now?
8. (Original post by AlmostNotable)
(d2y/dz2)(dy/dx)(cosx)-(dy/dz)(sinx), where is the mistake now?
By the chain rule:

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