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# Strange problem with an equation

Asked by Christian on 7 May 2013

So I'm working on this school project where we have to calculate T from an equation using fsolve. But Matlab won't use my equation, because T isn't estimated yet.

So when i run the script:

JacobFormula = ((Q)/(4*pi*T))*(log((2.25*T*t)/((r^2)*S)))-s;

The output is:

Undefined function or variable 'T'.

Which results that i can't go futher to find T. (All the other values are known)

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Answer by the cyclist on 7 May 2013

First, the way you have defined JacobFormula, it is a variable and not a function. Here is a simple example of defining a function:

```>> F = @(T) 3*T;
```

Then you can see that

` >> F(8)`
```ans =
```
`    24`

You will be able to use that function F as an argument to fsolve.

Christian on 7 May 2013

Oh thank you, that got me a little futher :-) But now it comes up with this:

Error using ^ Inputs must be a scalar and a square matrix. To compute elementwise POWER, use POWER (.^) instead.

Error in @(T)((Q)/(4*pi*T))*(log((2.25*T*t)/((r^2)*S)))-s

the cyclist on 7 May 2013

Can you post your full code, or at the very least a self-contained portion that can be run from scratch?