Betsy Thomas, age 13, of Glendale, Ariz., for her question:

JUST WHAT IS CALCULUS?

Calculus is one of the most important branches of mathematics. Calculus deals with changing quantities.

Students usually learn calculus in college after they have mastered algebra, plane geometry, trigonometry and analytic geometry. The word "calculus" comes from Latin and means "pebble." People once used pebbles to solve problems in mathematics.

Mathematicians usually call calculus "the calculus" to distinguish it from other methods of computation.

Calculus has two main branches: differential calculus and integral calculus. The central problem of differential calculus is to find the rate at which a known, but varying quantity changes. Integral calculus has just the reverse problem. It tries to find a quantity knowing the rate at which it is changing.

Since the development of calculus in the 1600s, mathematics has grown by leaps and bounds. Calculus introduced new methods into mathematics that stimulated much of this growth. Many branches of science and all branches of engineering use calculus to develop theories and to solve practical problems.