Some help with excel

[»Dav]

I am trying to get the formula from an exponential fit im my chart into a cell to use for a set of calculations i require.

 

I dont want to type it in because im trying to make an automated sheet for producing some calibration curves.

 

My formula from the test data set is this:

 

y = 33768*Exp(-3.7892*x)

 

I can get the first (33768) part with this formula =EXP(INDEX(LINEST(LN(y),x),1,2))

 

Y=Range of y valye

X=range of X values

 

The suggested formula for my second number (-3.7892) is this =INDEX(LINEST(LN(y),x),1) but this does not give me the correct number. (i get -875.51)

 

Anyone know the correct formula to use?

 

Thanks in advance

[Samapico]

Not sure what you're trying to do......

 

Are you trying to read the values of the formula from 'y = 33768*Exp(-3.7892*x)' as a string?

If so... why?

Why not just put the 33768 in one cell, and -3.7892 in another, and make a formula with these cells?

 

Or you have a bunch of formulas in that format and need to grab the values?

[»Dav]

solved it, was a problem with my cross sheet referencing.

 

The formula I needed was =LOGEST(y,x)

 

What I was trying to do was use the formula of the exponential to predict a value of Y where X is known

 

(if it means anything its gel filtration [aka size exclusion chromatography])

The reason I'm not just copying it down is because the calibration curve is unique to the piece of equipment being used, and it has to be calibrated with standards. As I will be using many of these things, and my co workers also use some of their own, I wanted to design a spreadsheet that allows you to input the calibration data and it will do the rest. Then on the main sheet you have a box into which you put your experimentally determined X value and it will predict Y.

 

You also have to re-calibrate these things quite frequently to keep them accurate so in the long run i wanted it to save me time as well.

 

I will be making another one for something else next week. Lucky for me its a simple linear regression so much more simple to do.