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# C3 Differentiation watch

1. Given that x = 4 sin(2y + 6), find dy/dx in terms of x.

Using the chain rule, I got dx/dy = 8cos (2y + 6) so dy/dx = 1/8cos (2y + 6).

From here:

Which is wrong, I'm pretty sure I used the identity wrong but i can't think of one that links the 8cos(2y+6) to x. Any ideas?
2. (Original post by fait)
[B]

Fixed.
3. Cheers.

Which is still wrong. I don't really see what I'm doing wrong though. =z

4. So, we can rearrange this to get:

(taking positive square root)

Hence, .

We can check this is right:

Differentiating, we get the same result:

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Updated: November 17, 2008
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