Just as real numbers include rational numbers and irrational numbers, complex numbers include real numbers and imaginary numbers.

Imaginary number is a number produced to meet the needs of mathematical development. The basic theorem is I 2 =- 1

The basic form of a complex number is a+bi, where b = 0 is a real number and a=0 is a pure imaginary number.

Calculate the expansion that satisfies the rational formula, (1+2i) 2 =1+4i+i2 = 4i,

Another example is: I 2 =-1,I 4 = (-1) 2 =1.