Solving system of 3 non-linear equations.

Hello, I'm trying to solve a system of equations using matlab.

The three variables are: xo2, xo, xar

I've entered the equations in as follows:

syms xo2 xo xar
eq1 = xo2 +xo +xar = 1
eq2 = 2*xo2 +xo -4*xar = 0
eq3 = 2.063E-4*xo2 = xo^2

Then, to solve the system for the variable xo I typed:

solve('eq1', 'eq2', 'eq3', xo)

and I get this message: Warning: Explicit solution could not be found.

What am I doing wrong? I'm fairly ceratain that this system is solvable.

is it because I am using symbolic algebra? For hte problem that I am solving, I dont need a general expression for the value, I just need a number.

[YARA NABA]

syms x y z
eq1=exp(x)+sqrt(y)-z.^3-2
eq2=x.^2-y-z-5
eq3=x+exp(y-1)+z-7
sol=solve(eq1,eq2,eq3)
sol

how can i rewrite it

[darova]

What it is wrong with it?

Answer 1

[Oleg Komarov]

Rewrite as:

syms xo2 xo xar
eq1 = xo2 +xo +xar - 1;
eq2 = 2*xo2 +xo -4*xar;
eq3 = 2.063E-4*xo2 - xo^2;

sol = solve(eq1,eq2,eq3);  

sol.xo

Oleg

[MJTHDSN]

Dear Oleg,

I have a similar question but a little bit confusing. Let`s assume we have 6 equations as below:

EQ1:a{{(L^2)*(Z^2)+(L^2)*(M^2)-2*L*(Z^2)+(Z^2)}}=(L^2)*(T^2)-2*L*(T^2)+ (T^2)-(2*L*T*B)+(T*B)+(B^2)

EQ2: b{{(L^2)*(Z^2)+(L^2)*(M^2)-2*L*(Z^2)+(Z^2)}}=(L^2)*(T^2)+(2*L*T*B)+(B^2)

EQ3: c{{(L^2)*(Z^2)+(L^2)*(M^2)-2*L*(Z^2)+(Z^2)}}=(T^2)+(2*T*B)+(B^2)

EQ4:d{{(L^2)*(Z^2)+(L^2)*(M^2)-2*L*(Z^2)+(Z^2)}}=(L^2)*(T^2)-2*L*(T^2)+ (T^2)-(2*L*T*B)-(T*B)+(B^2)

EQ5: e{{(L^2)*(Z^2)+(L^2)*(M^2)-2*L*(Z^2)+(Z^2)}}=(L^2)*(T^2)-(2*L*T*B)+(B^2)

EQ6: f{{(L^2)*(Z^2)+(L^2)*(M^2)-2*L*(Z^2)+(Z^2)}}=(T^2)-(2*T*B)+(B^2)

in the equations above a,b,c,d,e and f are the numerical known values (0.543 for example). So we have 6 equations with 5 unknowns as L, Z, M, T and B.

Can you please give me cues how to solve the equations to find these unknowns using MATLAB.

Best,

[Adithya Valavi]

Plz tell me if you knew how to solve this equations

[Christopher Creutzig]

Adithya Valavi, did you try vpasolve?

It's usually a good idea to post a new problem in a new question, rather than adding a comment to something related.

Answer 2

[Pierce Brady]

the output class will be syms, so try casting the answer to a double

class(ans)

double(ans)

class(ans)

[Tyler Tomlinson]

That was a huge help, thank you so much

Answer 3

[Pier Giorgio Petrolini]

I hope it can be already helpfull...

syms x y z
eq1=exp(x)+sqrt(y)-z.^3-2
eq2=x.^2-y-z-5
eq3=x+exp(y-1)+z-7

eqs = [eq1, eq2, eq3]
[x,y,z]=vpasolve(eqs,[x,y,z])

% Reported results
x = -2.8;
y =  3.33;
z = -0.48;

[Stephen Ofori]

This is also working, but the approximations makes it unsuitable for solutions where very small errors are required. If your work require minimal error then it is better to use the solve or fsolve.

Answer 4

[Tushar]

Ah thanks, excellent.

Now the only problem is that when I use your code, matlab spits out a crazy number like this:

ans =

 - 2995371679198674497313343284804110513^(1/2)/147573952589676412928 - 12685211008020935/147573952589676412928

how do I get it to something more readable.. like scientific notation or something

[Walter Roberson]

Use vpa() or double() to get the number in decimal format.

[Tushar]

Thanks Walter