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Thomson Brooks-Core
Complex Numbers
A straightforward approach to teaching complex numbers, this lesson addresses the concepts of complex numbers, polar coordinates, Euler's formula, De moivres Theorem, and more. It includes a practice problems set with odd answers...
Wolfram Research
Wolfram Math World: Triangle Inequality
This MathWorld site gives a good explanation of triangle inequality. The reader should be familiar with summation notation.
Wolfram Research
Wolfram Math World: Power Series
This site gives many different summation formulas and their expansions. Very Comprehensive site