SOLUTION: find the derivative {{{f(x)=8e^(-7^x)}}} {{{"f'(x)=?"}}} lost help please

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Question 202999: find the derivative
f%28x%29=8e%5E%28-7%5Ex%29
%22f%27%28x%29=%3F%22
lost help please

Answer by Edwin McCravy(20060) About Me  (Show Source):
You can put this solution on YOUR website!

f%28x%29=8e%5E%28-7%5Ex%29 

Let y+=+f%28x%29

y=8e%5E%28-7%5Ex%29 

Take natural logs of both sides:

ln%28y%29=ln%288e%5E%28-7%5Ex%29%29

Use the fact that ln%28uv%29=ln%28u%29%2Bln%28v%29

ln%28y%29=ln%288%29+%2B+ln%28e%5E%28-7%5Ex%29%29%29 

Use the fact that ln%28e%5Eu%29+=+u to rewrite the last term:

ln%28y%29=ln%288%29+%2B+%28-7%5Ex%29

ln%28y%29=ln%288%29+-7%5Ex

Now finally we can take the derivative implicitly.  

1. We use d%2Fdxln%28u%29=%22u%27%22%2Fu to take the 
derivative of the term on the left.

2. ln%288%29 is a constant so its derivative is 0.

3. We use d%2Fdxa%5Eu=a%5Eu%2Aln%28a%29%2A%22u%27%22 for the last term

So for ln%28y%29=ln%288%29+-7%5Ex we take the derivatives:

%22y%27%22%2Fy=0+-7%5Ex%2Aln%287%29%2A1

%22y%27%22%2Fy=-7%5Ex%2Aln%287%29

Multiply both sides by y

%22y%27%22=-7%5Ex%2Aln%287%29%2Ay

Use y=8e%5E%28-7%5Ex%29 to replace y

%22y%27%22=-7%5Ex%2Aln%287%29%2A%288e%5E%28-7%5Ex%29%29

Rearranging the factors:

%22y%27%22=-8%2A7%5Ex%2Ae%5E%28-7%5Ex%29%2Aln%287%29

Replace y by %22f%27%28x%29%22

%22f%27%28x%29%22=-8%2A7%5Ex%2Ae%5E%28-7%5Ex%29%2Aln%287%29

Edwin