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Massachusetts Institute of Technology
Mit: Calculus Index
This site from the Massachussets Institute of Technology has links to various topics in calculus.
Wolfram Research
Wolfram Math World: Multiple Angle Formulas
A site from Mathworld on multiple angle formulas. This site shows how to express sin(nx)in terms of sin(x) and cos(x) using the binomial theorem and Euler's formula. If you are not yet familiar with these formulas, check out the links to...
Cut the Knot
Cut the Knot: Existence of the Orthocenter
This site defines the altitude of a triangle and gives the standard notation for altitudes of a triangle. The site then goes to list several proofs involving altitudes and orthocenters.