Symbolic Toolbox Solving for Zeros

Hey! I've tried a few things but cant seem to get this script to solve for the x values of a functions values. Any ideas?

%% function

function [zeros] = dPlotInfo(func,range)

%

%This function plots a function and its 1st and 2nd derivatives

%It also returns any zeros

%

figure(1);

fplot(func,range,1);

hold on;

fplot(diff(func,1),range);

fplot(diff(func,2),range);

syms x;

zeros = fzero(func == 0 ,range);

%AHHHHH

end

%% test script used for the function

range = [-5 5];

syms x;

f = sin(3*x);

z = dPlotInfo(f, range);

disp(z);

Answer 1

[Paul]

Hi Ryan,

fzero is not used in the Symbolic Math Toolbox. Instead, check into using solve or vpasolve for this problem.

[Ryan Coder]

I did attempt to use solve before fzero but I couldn't get it to work, much less over a specific range. Could you give me an example?

[Paul]

Here is one way to use solve, taken nearly verbatim from its doc page

syms x real
f(x) = sin(3*x);

Solve the equation, parameterically if needed. If the solution(s) isn't parametertized, then it can be be checked manually to determine if it's in the desired range, which isn't the case here.

[solx,parameters,conditions] = solve(f(x),'ReturnConditions',true)

solx = 

parameters = 

k

conditions = 

Now solve for the parameter that forces the solution to lie within the desired range

assume(conditions)
restrictions = [solx > -5 , solx < 5];
solp = solve(restrictions,parameters)

solp = 

And sub those parameters back into the solution:

valx = subs(solx,parameters,solp)

valx = 

Answer 2

[Star Strider]

One approach —

range = [-5 5];
syms x;
f(x) = sin(3*x);
z = dPlotInfo(f, range);

Warning: The tolerance parameter input to fplot has been removed.

disp(z);

function [zeros] = dPlotInfo(func,range)
%
%This function plots a function and its 1st and 2nd derivatives
%It also returns any zeros
%
digits(5)                                               % Set Precision On Returned Symbolic Numbers (Convenience)
figure(1);
hfp = fplot(func,range,1);
hold on;
fplot(diff(func,1),range);
fplot(diff(func,2),range);
syms x;
zix = find(diff(sign(hfp.YData)));                      % Approximate Indices Of  Zero-Crossings
for k = 1:numel(zix)
    zeros(k) = vpasolve(func, x, hfp.XData(zix(k)));    % Use The X-Values For Each Approximate Index Value As A Starting Value
end
%AHHHHH
end

.