MATLAB: How to set an assumption on a symbolic variable in a symbolic expression

assumptionsymbolic

Hello!

I want to solve some equations using symbolic math toolbox.I only get some strings like '3*x1-2*x2'.x1,x2 should be positive numbers.

I can use str2sym(string) to get a symbolic expression 3*x1-2*x2.

Then, how to set an assumption on x1,x2 in the symbolic expression? The assumption is x1>0,x2>0.

Thank you in advance

Best Answer

Hi,

you can use the assume function for this purpose.

Note the section for multiple assumptions which requires assumeAlso to work properly.

Best regards

Stephan

Related Solutions

MATLAB: Is it possible to simplify symbolic expressions with known relations

I assume (link) it would!

syms x1 x2
assume(x1*x2 == 1)
a = x1*x2 + x1*x2;
b = 2*a

producing:

b =
4*x1*x2

MATLAB: Simplifying symbolic expressions using assumptions

I don’t think there is a way to directly approximate a symbolic expression in MATLAB with another symbolic term.

However, here is a workaround for the example you provided. Since x*y >> 1, you may want to consider replace (x*y+1) with x*y by using the function sub (See Documentation) .

For example, the following code can be executed

>> syms x y
>> A = x/(x*y+1);
>> A = subs(A, 'x*y+1', 'x*y');

The result will be A = 1/y.

-Yu

Best Answer

Related Solutions

MATLAB: Is it possible to simplify symbolic expressions with known relations

MATLAB: Simplifying symbolic expressions using assumptions

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