Tìm hàm phần thực và hàm phần ảo của hàm phức f(z) = z3 + 2z

Very interesting question!

But, √-4 is not a real number; in fact, the result of √-4 is 2i. 

The i is an imaginary number and it basically is a solution for √-1 

(So, i^2 is = -1).

Since -4 is negative, the square root doesn't exist (in terms of real numbers) just because you can't multiply any two identical numbers to get a negative result.

-2 x -2 = +4
2 x 2 = +4

That is why √-4 is NOT considered a real number.

Hope I could help you out!
Have a good one, and God bless!