Why do I get a fraction instead of a rational number when I solve two simple simultaneous equations

Question

Keith Jones 2016-10-2

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编辑： Walter Roberson 2020-12-21

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

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Answer 1

John D'Errico 2016-10-2

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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.

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Steven Lord 2020-12-20

在 MATLAB Online 中打开

If the expressions contain symbolic variables use vpa to display the approximation of the symbolic result to a specified number of decimal places.

If the expressions are symbolic representations of numbers (no symbolic variables) use double.

sqrt5 = sqrt(sym(5))

sqrt5 =

double(sqrt5)

ans = 2.2361

syms x

xsqrt5 = x*sqrt5

xsqrt5 =

vpa(xsqrt5)

ans =

Calling double on xsqrt5 won't work because of the presence of the x in the expression.

double(xsqrt5)
Error using symengine
Unable to convert expression containing symbolic variables into double array. Apply 'subs' function first to substitute values for variables.

Error in sym/double (line 702)
        Xstr = mupadmex('symobj::double', S.s, 0);