"I think, therefore I am." Wise words delivered from the French philosopher. Review the slideshow to get to know the contributions of this introduction of Rene Descartes.

This lesson demonstrates how to use triangles to determine the sum of the interior angles of a polygon. It is 1 of 5 in the series titled "Using Triangles to Find the Sum of Interior Angles."

A picture of the impressive Taj Mahal dominates the site, but there is also a brief video where our guide discusses the mysterious history and architecture. Clicking on "Explore the Topic" provides more information on the architecture....

Front cover of the novel Flipped by Wendelin Van Draanen.

This lesson provides practice problems that involve the sum of the interior angles of triangles. It is 5 of 6 in the series titled "Triangle Interior Angles Practice."

This lesson provides practice problems that involve the sum of the interior angles of triangles. It is 1 of 6 in the series titled "Triangle Interior Angles Practice."

This personal site provides a diagram and description of the addition of vectors. The triangle law of addition and the parallelogram law are given.

This site explains the infinite series through visualization.

The sum of any number and its additive inverse is always a zero. This site gives you the rule for this property.

This lesson teaches how to dissect complicated fractions into the sum of simple fractions.

This lesson teaches how to dissect complicated fractions into the sum of simple fractions.

This lesson teaches how to dissect complicated fractions into the sum of simple fractions.

This lesson teaches how to dissect complicated fractions into the sum of simple fractions.

In this lesson, special cases of factoring are described, including perfect squares, difference of squares, and difference and sum of cubes. Exercises with answers included.

In this lesson, special cases of factoring are described, including perfect squares, difference of squares, and difference and sum of cubes. Exercises with answers included.

In this lesson, special cases of factoring are described, including perfect squares, difference of squares, and difference and sum of cubes. Exercises with answers included.

In this lesson, special cases of factoring are described, including perfect squares, difference of squares, and difference and sum of cubes. Exercises with answers included.

A map from 1904 of the Panama Canal showing terrain relief of the isthmus, noting the 'Canal always follows the line of the Railroad’ from Colon to the city of Panama. "For generations the Panama Canal had been the dream of man and for...

Illustrated dictionary entry for Agamemnon sums up the principal events of his life in one short paragraph.

Illustrated dictionary entry for Agamemnon sums up the principal events of his life in one short paragraph.