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# 5: Complex Numbers and Polar Coordinates

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• 5.1: The Complex Number System
To make sense of solutions of quadratic equations that are not real, we introduce complex numbers. Although complex numbers arise naturally when solving quadratic equations, their introduction into mathematics came about from the problem of solving cubic equations.
• 5.2: The Trigonometric Form of a Complex Number
Multiplication of complex numbers is more complicated than addition of complex numbers. To better understand the product of complex numbers, we first investigate the trigonometric (or polar) form of a complex number. This trigonometric form connects algebra to trigonometry and will be useful for quickly and easily finding powers and roots of complex numbers.
• 5.3: DeMoivre’s Theorem and Powers of Complex Numbers
The trigonometric form of a complex number provides a relatively quick and easy way to compute products of complex numbers. As a consequence, we will be able to quickly calculate powers of complex numbers, and even roots of complex numbers.
• 5.4: The Polar Coordinate System
In our study of trigonometry so far, whenever we graphed an equation or located a point in the plane, we have used rectangular (or Cartesian) coordinates. The use of this type of coordinate system revolutionized mathematics since it provided the first systematic link between geometry and algebra. Even though the rectangular coordinate system is very important, there are other methods of locating points in the plane. We will study one such system in this section.
• 5.E: Complex Numbers and Polar Coordinates (Exercises)