Predict a value from an exponential function without using solver

[tml87]

Hello all,

In the attached spreadsheet, I would like the attached spreadsheet to give me a solvent volume (first column) for a viscosity of 400 cP (third column) when i change the inputs in Column R without using solver or graph trendlines to achieve it.

Any ideas?

Any and all help is appreciated.

Thanks,
Taher

[MrShorty]

2 possible approaches I can see:

1) A lot of times we use tools like Solver to avoid doing the algebra. The equation you have there is pretty ugly -- with all the parentheses and such -- but, at first glance, it looks to me like it should be possible to algebraically solve the equation for Vs. Then you will have an expression for Vs in terms of all of those other values, and can put 400 in for mu(p) and explicitly calculate Vs.

2) Solver, by default, uses a Newton Raphson type algorithm for these kinds of problems. If you don't want to do the algebra (or if I'm mistaken and it is impossible to solve explicitly for Vs), then you can program your own NR algorithm (or other favorite numerical algorithm. Given the table of values you have, it might be easier to program in a secant or bisection type method) into the spreadsheet and have it calculate Vs at a given mu(p).

[tml87]

Thanks for your reply! It is unfortunately not possible to explicitly solve for Vs. The equation has an "alpha" entity that is dependent on Vs (equations listed on the bottom of the spreadsheet)

I will try the bisection method, but I was wondering if there is not a formula or and excel function that does that?

Thanks again for your help!

- T

[MrShorty]

I will try the bisection method, but I was wondering if there is not a formula or and excel function that does that?

Solver/Goal Seek is the built in tool for solving equations numerically. When you decide not to use either of these tools, then you need to program it in yourself.