MATLAB: [Error using char] How to solve a transcendental equation for given variables

char

This is my code which enabled a graph to be plotted.

s = 'F/(11*((0.8*10^-3)^2)*2.41*10^9)= sin(x)*((cos (18.97)/cos (x))-1)^1.5';
ezplot(s , [5 10 15 20])

This is the code which gave the following error.

*Error using char Complex values cannot be converted to chars

Error in solve>getEqns (line 245) vc = char(v);

Error in solve (line 141) [eqns,vars,options] = getEqns(varargin{:});*

//Defining variables
a =5;
b=13;
c=1*10^-3;
d=0.57*10^-3;
e = 0.57*10^-3;
f=0.02*10^-3;
g = d/c+e/c-1;
h = acos(1 - f/(2*g*c));
i = 6*(10^6)*g^2+3*(10^6)*g*6894.75729;
//Transcendental function
k= '(a/(b*(c^2)*i) = (sin(j)*((cos(h))/(cos(j))-1)^1.5)';
ezplot(k, [5 10 15 20]);

The concern I have is the error only appears when I change the value within the function into a defined variable (e.g a,b,c)

What am I doing wrong and how do I correct this?

Best Answer

When you pass a string, values you have set in the workspace are not used. So the "i" you defined as a real value will not be paid attention to and instead "i" will have its default meaning as the imaginary constant, sqrt(-1)

Try

k= @(j) (a/(b*(c^2)*i) - (sin(j)*((cos(h))/(cos(j))-1)^1.5);

Related Solutions

MATLAB: Simplified solution to system of algebraic equations

Try this

syms a b x N g b L m
eqns = [(1/b)*(-2*N*sin(a))/3==m*L, x/b==-(5*L*cos(a))/6+(5*a^2*L*sin(a))/6, N/b==m*(5*L*sin(a))/6+(5*a^2*L)/(6*cos(a) +3*g)]; 
S = solve(eqns,[b m x]);
sol = S.x
sol_simplified = simplify(S.x)

Result:

sol =
-(18*N*cos(a)^2 + 10*N*cos(a)^2*sin(a)^2 + 9*N*g*cos(a) - 18*N*a^2*cos(a)*sin(a) - 5*N*a^2*g*sin(a)^3 + 5*N*g*cos(a)*sin(a)^2 - 10*N*a^2*cos(a)*sin(a)^3 - 9*N*a^2*g*sin(a))/(18*a^2)
 
sol_simplified =
(N*(cos(a) - a^2*sin(a))*(5*cos(a)^2 - 14)*(g + 2*cos(a)))/(18*a^2)

MATLAB: I cannot get the error.

a(x)=(1/2)*(cos(x)+3);

That defines a as being a symbolic function

a(b)= vpa(a(b))

That attempts to define the value of the function evaluated at b, but MATLAB does not permit you to define a general function and then assign values to the function at specific locations.

Below is the code rewritten in a way that is acceptable to MATLAB:

 syms f(x) a(x)
f(x)=(cos(x)-(2*x)+3);
a(x)=(1/2)*(cos(x)+3);
da=diff(a,x);
b=pi/2.5;
 s='a                    f(x)';
 
 disp(s);
 
for i=(1:100)
    
     if  abs(vpa(da(b)))<1  
    
    if vpa(f(a(b)))~=0
            
        a(x) = piecewise(x==b, vpa(a(b)), a(x));
        
        f(x) = piecewise(x == b, vpa(f(b)), f(x));
        
            A=[num2str(double(a(b))),'            ',num2str(double(f(b)))];
            
            
            disp(A);
            
           a(x)= piecewise(x==b, vpa(a(a(b))), a(x));
            
            i=i+1;
            
        elseif vpa(f(b))==0
           
            a(x)= piecewise(x==b, vpa(a(b)), a(x));
        
        f(x) = piecewise(x == b, vpa(f(b)), f(x));
            
            A=[num2str(doubble(a(b))),'               ',num2str(double(f(b)))];
            
          disp(A);
          
    end
    
     else 
         
         a(x) = piecewise(x==b, vpa(a(a(b))), a(x));
            
            i=i+1;
        
     end
    
end

I made no attempt at all to correct the algorithm: I just corrected the syntax to make it do exactly what you coded.

Best Answer

Related Solutions

MATLAB: Simplified solution to system of algebraic equations

MATLAB: I cannot get the error.

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