2. Generally, numbers in the form of z=a+bi (both a and b are real numbers) are called complex numbers. Where a is called the real part, b is called the imaginary part, and I is called the imaginary part. When the imaginary part b of z = 0, then z is a real number; When the imaginary part b≠0 and the real part a = 0 of z, z is often called pure imaginary number. Complex number field is an algebraic closure of real number field, that is, any polynomial with complex coefficients always has roots in complex number field.