Mark Ryan has more than three decades' experience as a geometry teacher and tutor. He has a gift for mathematics and a gift for explaining it in plain English. He tutors students in all junior high and high school math courses as well as math test prep, and he's the founder of The Math Center on Chicago's North Shore. Ryan is the author of Geometry For Dummies, Geometry Essentials For Dummies, Calculus For Dummies, and several other math books.