MATLAB Answers

Solving system of 3 non-linear equations.

1,216 views (last 30 days)
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.

  0 Comments

Sign in to comment.

Accepted Answer

Oleg Komarov
Oleg Komarov on 13 Feb 2011
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

  3 Comments

MJTHDSN
MJTHDSN on 12 Apr 2018
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
Adithya Valavi on 24 Mar 2020
Plz tell me if you knew how to solve this equations
Christopher Creutzig
Christopher Creutzig on 25 Mar 2020
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.

Sign in to comment.

More Answers (4)

Pierce Brady
Pierce Brady on 30 Mar 2011
the output class will be syms, so try casting the answer to a double
class(ans)
double(ans)
class(ans)

  1 Comment

Tyler Tomlinson
Tyler Tomlinson on 12 Dec 2015
That was a huge help, thank you so much

Sign in to comment.


Tushar
Tushar on 13 Feb 2011
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

  2 Comments

Walter Roberson
Walter Roberson on 14 Feb 2011
Use vpa() or double() to get the number in decimal format.

Sign in to comment.


YARA NABA
YARA NABA on 10 Mar 2019
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

  1 Comment

darova
darova on 10 Mar 2019
What it is wrong with it?

Sign in to comment.


Pier Giorgio Petrolini
Pier Giorgio Petrolini on 23 Mar 2020
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;

  0 Comments

Sign in to comment.

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!