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# C3 Differentiation Urgent Help watch

1. Given that y = ln(1+e^x), find that dy/dx and show that (1+e^x)(d^2y/dx^2) = dy/dx.

dy/dx = e^x/1+e^x

I found that the d^2y/dx^2 = e^x(1+e^x) - e^2x/ (1+e^x)^2.

Can't do the last part though - help please? Will rep
2. lnu=1/u
so ln(1+e^x) = 1/(1+e^x) multiplied by the derivative of 1+e^x which is e^x
so (1/(1+e^x))(e^x) = e^x/1+e^x
3. (Original post by TheKevinFang)
Given that y = ln(1+e^x), find that dy/dx and show that (1+e^x)(d^2y/dx^2) = dy/dx.

dy/dx = e^x/1+e^x

I found that the d^2y/dx^2 = e^x(1+e^x) - e^2x/ (1+e^x)^2.

Can't do the last part though - help please? Will rep
Multiply the 2nd derivative by , expand the brackets in the numerator and look for cancellation
4. (Original post by GUMI)
lnu=1/u
so ln(1+e^x) = 1/(1+e^x) multiplied by the derivative of 1+e^x which is e^x
so (1/(1+e^x))(e^x) = e^x/1+e^x
?????
5. (Original post by Indeterminate)
Multiply the 2nd derivative by , expand the brackets in the numerator and look for cancellation
https://onedrive.live.com/redir?resi...nt=photo%2cJPG

This is the best I could get - I'm not sure what to do from here, even if it may be painfully obvious...
6. (Original post by TheKevinFang)
https://onedrive.live.com/redir?resi...nt=photo%2cJPG

This is the best I could get - I'm not sure what to do from here, even if it may be painfully obvious...
Noo, don't cancel the brackets containing .

You should expand the numerator and look for two terms that cancel each other out!
7. (Original post by TheKevinFang)
Given that y = ln(1+e^x), find that dy/dx and show that (1+e^x)(d^2y/dx^2) = dy/dx.

dy/dx = e^x/1+e^x

I found that the d^2y/dx^2 = e^x(1+e^x) - e^2x/ (1+e^x)^2.

Can't do the last part though - help please? Will rep
Expand the numerator of your second part and simplify. Then multiply (1+e^x) with the second part, then cancel and you'll get your answer.
8. Moved to maths.
9. (Original post by TheKevinFang)
Given that y = ln(1+e^x), find that dy/dx and show that (1+e^x)(d^2y/dx^2) = dy/dx.

dy/dx = e^x/1+e^x

I found that the d^2y/dx^2 = e^x(1+e^x) - e^2x/ (1+e^x)^2.

Can't do the last part though - help please? Will rep
Is this parametric differentiation or am I completely off the hook?
10. Am I too late for this thread?
11. (Original post by TeeEm)
Am I too late for this thread?
No feel free to go through it as I'm confused as to what the solution is
12. (Original post by RosaA)
No feel free to go through it as I'm confused as to what the solution is
I would normally but I am brain dead now.
13. (Original post by TeeEm)
I would normally but I am brain dead now.

;'D
14. (Original post by Indeterminate)
Noo, don't cancel the brackets containing .

You should expand the numerator and look for two terms that cancel each other out!
Thanks got it - the e^x's cancel out

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