Equation: knowing y - wanting x..

  • Thread starter Thread starter ruter
  • Start date Start date
R

ruter

Hello.

I have the folloving equation:
y=a*x^b/(c^b+x^b)
a, b and c are all constants.

My problem is that I know y. (y=50) Howcan I get hold of an -exact- X
Can excel do this automaticly or do I have to slove the equatio
regarding to x?

Thank you.

If excel can't do this, I could need som help solving the equation
x=...
 
Hi
Excel can't do this automatically. But you could use the Solver Add-in
to get a solution for 'x'.
 
Excel can't do this automatically. But you could use the Solver Add-in
to get a solution for 'x'. ...
Click to expand...
...

No, Excel can't do this, but symbolic math packages could. For that matter, all
it takes is a bit of algebra.

y = a * x^b / (c^b + x^b)

y = a / ((c / x)^b + 1)

((c / x)^b + 1) * y = a

(c / x)^b + 1 = a / y

(c / x)^b = (a / y) - 1

(x / c)^-b = (a / y) - 1

x / c = ((a / y) - 1)^(-1 / b)

x = c * ((a / y) - 1)^(-1 / b)

When an algrbraic solution is possible, it's generally better than using Solver.
 
Harlan Grove wrote:
[...] something reminding me distantly at my math classes :-)
When an algrbraic solution is possible, it's generally better than
using Solver.
Click to expand...

totally agree with you on that!
Regards
Frank
 
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