# Cant make my syms function into a fsolve function

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Mikkel Ibsen on 4 Dec 2017
Commented: Walter Roberson on 4 Dec 2017
Hi
Since I've found out that:
syms
Doesn't work in matlab gui, I need to change my code, and Im totally new at the fsolve function, and dont quite how it works, do I need to make a function file to make it work, I've look at many tutorials from mathworks but I still can't make my script work. This is what I need to make into a fsolve function:
example:
L = 12;
A = 0.3;
vaegt = 60;
Rcd = 250;
slag = (1:80)';
test = zeros(size(slag));
syms x
for h = 1:length(slag)
S0 = sqrt((2*x*vaegt*L)/(A*20000000));
kraeft = @(x)((1/1.5).*vaegt.*x./((0.2./slag(h))+0.5*S0))./1.3;
test(h) = solve(kraeft(x)==(Rcd),x);
end
If someone also see a way to make it calculate faster that would be nice :).

Walter Roberson on 4 Dec 2017
There is no problem using syms in a MATLAB GUI that is running interactively. There is only a problem if you attempt to compile or generate code.
Your system is effectively a quadratic, and has a pair of solutions:
(39/32000000000)*Rcd*(39*Rcd*slag*L +/- 39^(1/2)*(Rcd*slag*L*(39*L*Rcd*slag+640000000*A))^(1/2)+320000000*A)/(A*slag*vaegt)
There is no need to fsolve() at all.
Walter Roberson on 4 Dec 2017
>> syms L A vaegt Rcd slag x real
>> S0 = sqrt((2*x*vaegt*L)/(A*20000000));
>> S0
S0 =
((L*vaegt*x)/(10000000*A))^(1/2)
>> kraeft = @(x)((1/1.5).*vaegt.*x./((0.2./slag)+0.5*S0))./1.3;
>> solve(kraeft(x)==(Rcd),x)
Warning: Possibly spurious solutions.
> In symengine
In solve (line 293)
Warning: Solutions are valid under the following conditions: slag*((39*L*Rcd*(320000000*A + 39*L*Rcd*slag -
390000000*Rcd*slag*((L*(640000000*A + 39*L*Rcd*slag))/(3900000000000000*Rcd*slag))^(1/2)))/(320000000000000000*A^2*slag))^(1/2) ~= -2/5 & 0
<= L*Rcd*slag*(640000000*A + 39*L*Rcd*slag);
slag*((39*L*Rcd*(320000000*A + 39*L*Rcd*slag + 390000000*Rcd*slag*((L*(640000000*A +
39*L*Rcd*slag))/(3900000000000000*Rcd*slag))^(1/2)))/(320000000000000000*A^2*slag))^(1/2) ~= -2/5 & 0 <= L*Rcd*slag*(640000000*A +
39*L*Rcd*slag). To include parameters and conditions in the solution, specify the 'ReturnConditions' value as 'true'.
> In solve>warnIfParams (line 508)
In solve (line 357)
ans =
(39*Rcd*(320000000*A + 39*L*Rcd*slag - 390000000*Rcd*slag*((L*(640000000*A + 39*L*Rcd*slag))/(3900000000000000*Rcd*slag))^(1/2)))/(32000000000*A*slag*vaegt)
(39*Rcd*(320000000*A + 39*L*Rcd*slag + 390000000*Rcd*slag*((L*(640000000*A + 39*L*Rcd*slag))/(3900000000000000*Rcd*slag))^(1/2)))/(32000000000*A*slag*vaegt)
At the moment I am not sure why the stepwise approach gives only one answer.