Complex_number is a complex number whose cosine is to be calculated.
A complex number is a string expression resulting in the form "a+bi" or "a+bj", where a and b are numbers.
If the complex number is actually a real number (b=0), then it can be either a string expression or a number value.
The function always returns a string representing a complex number.
If the result is a complex number with one of its parts (a or b) equal to zero, that part is not displayed.
returns -0.416146836547142 as a string. The imaginary part is equal to zero, so it is not displayed in the result.