# Thread: Finding function from a graph

1. ## Finding function from a graph

I need to find a function from these numbers:

How could I do that?

2. Do you have an underlying model for the data? Or do you not have any clues as to the type of function the data is supposed to represent?

3. All I know about this is that this is humidity on the x axis and resistance in k ohms on the y axis. These numbers are almost linear on a semi log (if i'm not mistaken thats how it is called) scale.

4. Ah. That doesn't surprise me in the least. And yes, that is a semilog plot. Probably the easiest thing to do is to fit a straight line to the semilog plot of your data. The idea is this: assume your function (on the original plot) is of the form

$\displaystyle y=A\,e^{-k t}.$

Here it is essential that both $\displaystyle A>0$ and $\displaystyle k>0.$

Now, plotting in a semilog plot is the same as taking the logarithm of both sides of this equation. The result is the following:

$\displaystyle \ln(y)=\ln(A)+\ln(e^{-kt})=\ln(A)-kt.$

Your semilog plot has essentially plotted $\displaystyle \ln(y)$ versus $\displaystyle t.$ Therefore, you can find the slope and intercept of a straight-line-fit. The slope will be $\displaystyle -k$, and the intercept $\displaystyle \ln(A).$

Make sense?

5. Thank you. Solved my problem. The graph itself seems to be not a perfect exponent, because my solution does not correspont to the given graph perfectly. But thats not a problem in this case.

Thank you again.

6. You're welcome. Have a good one!