Complex numbers, a hybrid of the imaginary and the real, are the pinnacle of number systems.

The series moves on to high school math, specifically algebra and formulas.

This week, division - where many students hit the mathematical wall - is made less confusing.

The disturbing concept of subtraction, and how we deal with the fact that negative numbers are so . . . negative.

Treating numbers concretely - think rocks, for instance - can make calculations less baffling.

A debut column on math features an introduction to numbers, from upsides (they're efficient) to down (they're ethereal).

features 20 primary documents and resources for learning about the history and achievements of America's oldest and largest civil rights organization, the National Association for the Advancement of Colored People (NAACP). Topics include Booker T. Washington, voting rights, Brown v. Board of Education, Marian Anderson singing at the Lincoln Memorial, and more. (Library of Congress)

features classic books online for children and teens. Find the text and illustrations of "Humpty Dumpty," "Mother Goose," "The Three Bears," "Our Flag," "The Arabian Nights," "The Secret Garden," "A Christmas Carol," "The Raven," and other classics. The site also provides webcasts of authors, writing contests, and teaching resources. (Library of Congress)