SOLUTION: A = ln(x)-Bx Express x in terms of A and B. Please help.... thanks in advance.....

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Question 433406: A = ln(x)-Bx
Express x in terms of A and B.
Please help.... thanks in advance.....

Answer by richard1234(7193) About Me  (Show Source):
You can put this solution on YOUR website!
A+=+ln%28x%29+-+ln%28e%5E%28Bx%29%29+=+ln%28x%2F%28e%5E%28Bx%29%29%29

e%5EA+=+x%2Fe%5E%28Bx%29

e%5EA+=+xe%5E%28-Bx%29 Multiply both sides by -B

-Be%5EA+=+-Bxe%5E%28-Bx%29

There is a certain function called the Lambert W-function defined as the inverse of the function f%28z%29+=+ze%5Ez. Here, we have f%28-Bx%29+=+-Bxe%5E%28-Bx%29 so it follows that if we take the W-function of both sides,

-Bx+=+W%28-Be%5EA%29

x+=+%28-1%2FB%29W%28-Be%5EA%29

This is pretty much the simplest form that one can express x in, even though the W-function is somewhat obscure. There is, of course, using Newton's method, but it is very unlikely that a real number will be obtained.

Also, I checked my answer on Wolfram Alpha by putting arbitrary values for A and B (e.g. A = 253, B = 19), and it came out with the solution x+=+%28-1%2F19%29W%28-19e%5E253%29.

http://www.wolframalpha.com/input/?i=253+%3D+ln%28x%29+-+19x

Wikipedia article about the W-function:
http://en.wikipedia.org/wiki/Lambert_W_function