MATLAB: Do I get a fraction instead of a rational number when I solve two simple simultaneous equations

MATLABsystems of linear equations

These are the two equations I need to solve; for A and B, to calculate the viscosity of hydraulic fluid between to temperatures. The results are correct, but why does Matlab show them as fractions and not rational numbers?

syms A B
eqn1=A-520*B==3.7612;
eqn3=A-560*B==2.3026;
[X,Y] = equationsToMatrix([eqn1, eqn3], [A, B])
Z = linsolve(X,Y)
Z =
    22723/1000
   7293/200000
>> 22723/1000
ans =
     22.7230
>>  7293/200000
ans =
      0.0365

Best Answer

Because you are (for some irrational reason) using symbolic computations to solve a simple linear system of equations. Sort of equivalent to the use of a Mack truck to bring a single pea to Boston - massive overkill. Therefore the result is a symbolic one, and is left in symbolic form.

If you want to convert it to a floating point number, then use vpa, or use double.

Related Solutions

MATLAB: Solving Diophantine Equations in MATLAB

Use the “solve” function. Before using “solve”, assume that the variables in the Diophantine equation are integers to ensure “solve” returns integer solutions. For example, solve the linear Diophantine equation 1027*x + 712*y = 1;

For MATLAB R2015a and later:

syms x y integer                % assume ‘x’ and ‘y’ are integers

eqn = 1027*x + 712*y == 1;       % declare the equation

[xSol, ySol] = solve(eqn,[x y])  % solve for ‘x’ and ‘y’

xSol =
-165
ySol =
238

For MATLAB R2014b or before:

syms x y
assume([x y], 'integer')        % assume ‘x’ and ‘y’ are integers
eqn = 1027*x + 712*y == 1;       % declare the equation
[xSol, ySol] = solve(eqn,[x y])  % solve for ‘x’ and ‘y’
xSol =
-165
ySol =
238

For details on “solve”, see: https://www.mathworks.com/help/symbolic/solve.html

For more information on assumptions, see: https://www.mathworks.com/help/symbolic/assumptions-for-symbolic-objects.html

MATLAB: Does the SUBS function in the Symbolic Toolbox give an output of type DOUBLE

This is a designed feature of the SUBS function. To ensure that your output remains a symbolic object, use the SYM function:

syms A x y
A = sym(1);
class(A)
A = sym(subs(A,x,y));
class(A)

Best Answer

Related Solutions

MATLAB: Solving Diophantine Equations in MATLAB

MATLAB: Does the SUBS function in the Symbolic Toolbox give an output of type DOUBLE

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