Solving an equation

**sydmil** · 05-12-2009, 10:10 AM

Hello,

I was wondering if there is a way to solve equations using XL.

I have a number of equations that look like this:

Y = 1.05007 + 0.948061 * Ln( ( X - 2.75394 ) / ( 3.07675 - X ) )

I know the value of Y (eg 3.5) and I need to find the X value that will give me the known Y value.

Any suggestions ?

THANKS!

**martindwilson** · 05-12-2009, 10:19 AM

first rearrange it to show x=blah blah y

**zbor** · 05-12-2009, 10:23 AM

If I did it correctly...

Book1.xls

**sydmil** · 05-12-2009, 10:28 AM

Zbor,

yeah, this is it ! man, you are fast... thank you very much !!

**shg** · 05-12-2009, 10:50 AM

That's a very fussy function -- it is defined only over a very limited domain. I used a plot to choose constraints, and Solver to get the answer (X=3.0540998180399). You can do Tools > Solver to see the setup.

**zbor** · 05-12-2009, 01:32 PM

You are right... It must be + and not - in B5-B1)/B2) +B3)/(1+

...and about domain check you can put something like:

=IF(B7=B4;"Can't divide by 0";IF((B7-B3)/(B4-B7)<=0;"Wrong domain";"OK"))

It looks two things:
X can't be same as 3.07675 due to divide /0
Number under LN must be greather than 0