# MandysNotes

## Complex Numbers (4)

Sunday, 01 May 2011 00:00

### A Brief Introduction to Complex Numbers

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Take a pencil and place it along the edge of a ruler, so that the ruler and the pencil are parallel, and so that the end of the eraser is at zero, and the point of the pencil points towards increasing numbers on the ruler.

Sunday, 01 May 2011 17:14

### Complex Numbers and Quadratic Polynomials

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Recall that a polynomial of degree two is called a Quadratic Polynomial.

A general quadratic polynomial (over the real numbers) is of the form:

$az^{2} + bz + c = 0.$

The formula for solving for the roots of this equation is:

$z = \frac{-b \pm \sqrt{b^{2} -4ac}}{2a} .$

Sunday, 01 May 2011 20:58

### The Complex Conjugate

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Two complex numbers of the form:

$z_+ = x + iy ,$

and,

$z_- = x - iy ,$

are called Complex Conjugates.

Sunday, 01 May 2011 23:16

### The Modulus of a Complex Number

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For any complex number $z = x + iy ,$

$z \bar{z} = (x + iy)(x -iy)$

$= x^{2} + ixy - ixy + y^{2} = x^{2} + y^{2} .$

That is, any complex number times its complex conjugate equals the square of its real part plus the square of its imaginary part.

Because both the real and imaginary parts of any complex number are real:

$z \bar{z} = x^{2} + y^{2} \geq 0, \forall z \in \mathbb{C}.$

We define the positive square root of $z \bar{z}:$

$+\sqrt{z\bar{z}} : = |z| ,$

to be the Modulus of the complex number $z .$