ln() is the natural log of a number. It's the exponent that you raise e (2.718
approx) to go get that number.

=ln(2.718) = about 1
since
e^1 is about 2.718

ln(100) = 4.60517
since
e^4.60517 (or 2.718 ^ 4.60517) = 100.

Some info for ln() and exp():
http://en.wikipedia.org/wiki/Natural_logarithm
http://en.wikipedia.org/wiki/E_%28ma...al_constant%29
http://en.wikipedia.org/wiki/Exponential_function


Starting with:
y = .8065 * ln(x) + 3.4412
subtracting 3.4412 from both sides:

y-3.4412 = .8065 * ln(x)
dividing both sides by .8065

(y-3.4412)/.8065 = ln(x)
Using each side as an exponent with e the base

exp((y-3.4412)/.8065) = exp(ln(x))

But exp(ln(x)) = x

so
exp((y-3.4412)/.8065) = x

So if you put the value for Y in A1, you could use this formula in B1:
=exp((a1-3.4412)/.8065)
to find what X is.


dr chuck wrote:

This is a log trendline formula from Excel semilog graphing.

y = 0.8065Ln(x) + 3.4412

can anyone tell me how the " 0.8065Ln(x) " part of this formula works?

I know y and i am solving for "x"

not sure the significance of the "Ln"

do i have to use some "log" function?

--
dr chuck

--

Dave Peterson