A complex number is a number consisting of a real part and an imaginary part. Complex numbers extend the idea of the one-dimensional number line to the two-dimensional complex plane by using the number line for the real part and adding a vertical axis to plot the imaginary part.

A complex number can be visually represented as a pair of numbers forming a vector on a diagram called an Argand diagram, representing the complex plane. Re is the real axis, Im is the imaginary axis, and i is the square root of -1.

http://en.wikipedia.org/wiki/Complex_number

In mathematics, a quadratic equation is a polynomial equation of the second degree. The general form is

where x represents a variable or an unknown, and a, b, and c are constants with a ≠ 0. (If a = 0, the equation is a linear equation.)

The constants a, b, and c, are called respectively, the quadratic coefficient, the linear coefficient and the constant term or free term. The term "quadratic" comes from quadratus, which is the Latin word for "square". Quadratic equations can be solved by factoring, completing the square, graphing, Newton's method, and using the quadratic formula

A quadratic equation with real or complex coefficients has two solutions, called roots. These two solutions may or may not be distinct, and they may or may not be real.

The roots are given by the quadratic formula

where the symbol "�" indicates that both

are solutions of the quadratic equation.

(Our solved example in mathguru.com uses this concept)

Discriminant

In the above formula, the expression underneath the square root sign is called the discriminant of the quadratic equation, and is often represented using an upper case Greek delta

A quadratic equation with real coefficients can have either one or two distinct real roots, or two distinct complex roots. In this case the discriminant determines the number and nature of the roots. There are three cases:

•       If the discriminant is positive, then there are two distinct roots, both of which are real numbers:

For quadratic equations with integer coefficients, if the discriminant is

perfect squares, then the roots are rational numbers-in other cases they may be quadratic irrationals.

•       If the discriminant is zero, then there is exactly one distinct real root, sometimes called a double root:

•       If the discriminant is negative, then there are no real roots. Rather, there are two distinct (non-real) complex roots, which are complex conjugates of each other:

where i is the imaginary unit.

Thus the roots are distinct if and only if the discriminant is non-zero, and the roots are real if and only if the discriminant is non-negative.

(Our solved example in mathguru.com uses this concept)

http://en.wikipedia.org/wiki/Quadratic_equation

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