Adversity Testing MATLAB. Why does matlab say this?

I give this command and get answer as below. What does this mean? I was simply doing adverstity testing of MATLAB.

 syms x
 s=solve(sin(x)^2 + cos(x)^2 == 1)

 s =
   C_

Answer 1

[David Young]

C_ is the set of complex numbers. That looks right, since the identity is true for any value of x.

[Seetha Rama Raju Sanapala]

How did you know about C_? Where can I get this information?

[David Young]

Note that in my answer, C_ is a link to the documentation. I found it by Googling "matlab symbolic toolbox C_".