Recall that a real number is a point on the real number line. Just as a real number should be considered as a point on the line, a complex number is considered a point in the plane which can be identified in the usual way using the Cartesian coordinates of the point. Thus
Multiplication and addition are defined in the most obvious way subject to the convention that i^{2} = −1. Thus,

and

You should prove the following theorem.
Theorem 1.5.1 The complex numbers with multiplication and addition defined as above form a field satisfying all the field axioms. These are the following list of properties. In this list, F is the symbol for a field.
When you have a field F some things follow right away from the above axioms.
Theorem 1.5.2 Let F be a field. This means it satisfies the axioms of the above theorem. Then the following hold.
Proof: Consider the first claim. Suppose

and so sure enough, there is only one such additive identity. Consider uniqueness of −x next. Suppose y is also an additive inverse. Then

so the additive inverse is unique also.

Now add −0x to both sides to conclude that 0 = 0x. Next

and by uniqueness of −x, this implies

so y = x^{−1}. ■
Something which satisfies these axioms is called a field. Linear algebra is all about fields, although in this book, the field of most interest will be the field of complex numbers or the field of real numbers. You have seen in earlier courses that the real numbers also satisfy the above axioms. The field of complex numbers is denoted as ℂ and the field of real numbers is denoted as ℝ. An important construction regarding complex numbers is the complex conjugate denoted by a horizontal line above the number. It is defined as follows.

What it does is reflect a given complex number across the x axis. Algebraically, the following formula is easy to obtain.
Definition 1.5.3 Define the absolute value of a complex number as follows.

Thus, denoting by z the complex number z = a + ib,

Also from the definition, if z = x + iy and w = u + iv are two complex numbers, then
Notation 1.5.4 Recall the following notation.

There is also a notation which is used to denote a product.

The triangle inequality holds for the absolute value for complex numbers just as it does for the ordinary absolute value.
Proof: Let z = x + iy and w = u + iv. First note that

and so



so this shows the first version of the triangle inequality. To get the second,

and so by the first form of the inequality

and so both
With this definition, it is important to note the following. Be sure to verify this. It is not too hard but you need to do it.
Remark 1.5.6 : Let z = a + ib and w = c + id. Then
For example, consider the distance between