# How to Input a function equation in matlab

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Ivy Shen on 3 Oct 2018
Edited: Stephen Cobeldick on 3 Oct 2018
Hi, I have an equation as following:
F - hm*(a-b)*log(1+F/(hm*(a-b))) = kt
I am trying to use fzero to solve for F in matlab, but I don't know how to input this equation in matlab first?

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KSSV on 3 Oct 2018
Read the documentation.....it got lot of example.....MATLAB is rich in documentation.
Ivy Shen on 3 Oct 2018
I am trying to write a function as following:
function F = GreenAmp(Ks,hm,t,a,b)
F - hm*(a-b)*log(1 + F/(hm*(a-b))) = Ks*t;
And I don't know what's the next step. Is there any easier way to do it? Can you specify which documentation I should look at?
Thank you.
Stephen Cobeldick on 3 Oct 2018
function F = GreenAmp(Ks,hm,t,a,b)
F - hm*(a-b)*log(1 + F/(hm*(a-b))) = Ks*t;
That will not work:
• variable F is used but undefined. In order to solve for F, F must be one of the input arguments to the function.
• the LHS of the = sign is not valid for assignment: the LHS should be one variable, which is the value of the function when it is evaluated.
"Can you specify which documentation I should look at?"

Stephen Cobeldick on 3 Oct 2018
Edited: Stephen Cobeldick on 3 Oct 2018
>> b = 1;
>> a = 2;
>> hm = 0.5;
>> kt = 0.9;
>> fun = @(F) F - hm*(a-b)*log(1+F/(hm*(a-b)));
>> y = fzero(@(x)fun(x)-kt,0)
y = 1.6230
>> fun(y)
ans = 0.90000
Note that this curve might have multiple solutions, depending on the parameter values: fzero only finds one solution, so you will have to adjust its starting value to find any other solutions. You should always plot the curve and check if the solution is reasonable:
X = -10:0.1:10;
Y = arrayfun(fun,X);
plot(X,Y)
hold on
plot(y,0.9,'*')

#### 1 Comment

KSSV on 3 Oct 2018
I have deleted my answer..which was ridiculous.

R2016b

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