Why is it wrong: e3=sym('si​n(x)^2+cos​(x)=0')

Why is it wrong: e3=sym('sin(x)^2+cos(x)=0')

Answer 1

[madhan ravi]

sym('') was replaced by str2sym('') in latest versions and you seem to be using 2018b version

e3=str2sym('sin(x)^2+cos(x)=0')
%  ^^^^^^^

[zhenyu zeng]

Why can not I input e3=sym('sin(x)^2+cos(x)=0')

[madhan ravi]

See the reason in the edited answer.

[zhenyu zeng]

But I can input sym('a') not need to input str2sym('a'). Why?

[madhan ravi]

Read the the error message clearly MATLAB states it clearly already:

Error using sym>convertChar (line 1459)
Character vectors and strings in the first argument can only
specify a variable or number. To evaluate character vectors and
strings representing symbolic expressions, use 'str2sym'.

Error in sym>tomupad (line 1225)
        S = convertChar(x);

Error in sym (line 214)
                S.s = tomupad(x);

[madhan ravi]

Post this information in the question you recently asked. This is completely irrelevant to this question.

[zhenyu zeng]

>> clear

>> sym('a')

ans =

a

[madhan ravi]

Did you read https://in.mathworks.com/matlabcentral/answers/457777-why-is-it-wrong-e3-sym-sin-x-2-cos-x-0#comment_696787 ? What did you understand?

[zhenyu zeng]

I can use sym('a'). Why can not I use sym('a+b')？

[zhenyu zeng]

Why not use str2sym('a') instead of sym('a’)?

[madhan ravi]

It only works for only a number or a variable. Read the documentations. Take MATLAB on-ramp course to learn the basics of MATLAB.

Answer 2

[KSSV]

syms x
e3 = sin(x)^2+cos(x)==0