Using 2 equations to solve a value

**jackleesteere** · 03-05-2014, 09:30 PM

Hi. Can some one please help me with this problem (see attached).
I have 2 equations to solve K, that use 4 inputs (A,B,D,H). A,B and D are known. H is unknown. How do i solve for H in excel? I believe a macro is required?
Many thanks

**Pete_UK** · 03-05-2014, 09:43 PM

So you have two equations:

K = (H*A)/(B-H)

and

K = H*D

The solution occurs when K = K, i.e.:

(H*A)/(B-H) = H*D

You need to rearrange this equation to get it in the form H = something. Simple algebra will get you to:

H = B - A/D

For the values given, H = 1.667 and K = 5

No need for a macro (or even for Excel !!)

Hope this helps.

Pete

**jackleesteere** · 03-05-2014, 10:05 PM

Thanks Pete.
The example above is a very simplified version of a series of problems i need to solve. I was hoping to use excel to solve it so i can aply it to these.

**MrShorty** · 03-05-2014, 10:55 PM

I find that, before I can work out the computer/Excel algorithm for something like this, I first need to understand the math. Even if your actual problem is more complex than the example given, can we still assume that it is a system of "linear" equations? Or do you actually have a system of non-linear equations? The example you gave was a system of 2 equations and 2 unknowns, is this representative of your actual problem or do you have more equations in more unkowns?

Assuming a system of linear equations, this tutorial should help you remember the techniques used to solve systems of linear equations. http://www.purplemath.com/modules/systlin1.htm

If they are non-linear, she also has a tutorial for those: http://www.purplemath.com/modules/syseqgen.htm

More complex systems of equations will require numerical methods (usually using the Solver utility that comes with Excel).

If the example you gave is representative of the type of problem you have, then the algebraic technique Pete_UK suggested should be sufficient, and is not difficult to program into Excel. If the example you gave is not representative of your problem, we will probably have to have a good example to make specific suggestions towards developing an algorithm.