Solve returning root but doesn't give a numerical expression.

Hello ! I'm actually using the function " solve ", but instead of returning an expression, i get the following : "

                                                 9
 root(z^4 - (10*z^3)/9 - z^2/6561 - z/729 - 91/6561, z, 1)
 root(z^4 - (10*z^3)/9 - z^2/6561 - z/729 - 91/6561, z, 2)
 root(z^4 - (10*z^3)/9 - z^2/6561 - z/729 - 91/6561, z, 3)
 root(z^4 - (10*z^3)/9 - z^2/6561 - z/729 - 91/6561, z, 4)

These are indeed the solutions, but why it doesn't express it ? If i use the function " roots " with a vector with the above coefficient, it gives me the solution numerically. Anyone have a way to either get the answer numerically, or to extract the coeffcient of the above expression automatically ? In any case, here is the simple code :

clear all
syms a 
b=9;h=0.1;
t2=a^4*b^3-b-h*(b^3-b^5*a^4)-(b^4*a^3-a-h*(a^3-b^4*a^5))

sol=solve(t2,a)

Thanks !

Answer 1

[Walter Roberson]

sol = solve(t2, a, 'MaxDegree', 4)

You should consider the possibility that you are using the wrong tool. solve() is for finding exact solutions. Your h is a floating point number rather than being 1/sym(10), which suggests that you are not interested in exact solutions. You should be giving consideration to using vpasolve() instead of solve()

[Boris Huljak]

Thank you very much !

Answer 2

[pepe]

I have had a similar version dependent exprience. My code is:

syms x

solve(x^3+x-1==0)

In Matlab 2019 it produces the useless root() output that you also mentioned. But in MATLAB 2014 it produces a nicer result comprised of fractions and sums of numbers;...so I recommend you also give it a try using an old version of MATLAB.

good luck

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