How to fix the large and mad fractions when dealing with symbolic vars
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Mohd Aaquib Khan
el 18 de Sept. de 2022
Comentada: Mohd Aaquib Khan
el 19 de Sept. de 2022
Hi guys,
I face this often, as you can see below that cos(phi) and cos(psi) are decent entities, but when they are used with a symbolic variable (kn here), it shows huge fractions. Is there an easy way to make the coefficients of k1 k2 and k3 in the short format or some decent fractions.
Even something as trivial as k1*cos(pi/2) is not evaluated to be 0 as shown below. I fear that this leads to truncation errors.
Thanks in advance.
cos(pi/2)
syms a
a*cos(pi/2)
4 comentarios
Torsten
el 18 de Sept. de 2022
I could only see your screenshots after I opened your contribution using "Edit".
Respuesta aceptada
Walter Roberson
el 19 de Sept. de 2022
The answer you are looking for is
digits(4)
sympref('FloatingPointOutput',true);
However, I do not recommend this.
1 comentario
Más respuestas (2)
Paul
el 18 de Sept. de 2022
In this expression
syms a
a * cos(pi/2)
the cos() is evaluated numerically and the result converted to a sym to multiply with a. The numerical cos(pi/2) is not zero.
Force the cos to be evaluated symbolically
a * cos(pi/sym(2))
or
a * cos(sym(pi)/2)
3 comentarios
Torsten
el 19 de Sept. de 2022
For phi = 30 and phi = 90, cos(phi) is correctly simplified.
What would you expect for cos(50) and cos(25) ? I don't see a simple representation.
Walter Roberson
el 19 de Sept. de 2022
You should never eval() a symbolic expression. eval() of a symbolic expression is not documented by Mathworks. In practice it is treated as eval(char()) of the expression. However, char() of a symbolic expression is written in a mix of MATLAB, English, and the internal MuPAD programming language, and is not generally something that can be computed.
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