Book excerpt: Learn and practice essential geometry skills. The answer to every problem, along with helpful notes, can be found at the back of the book. This volume focuses on fundamental concepts relating to triangles, and also covers quadrilaterals and other polygons. Topics include: lines, angles, and transversals; angles of a triangle; congruent triangles; similar triangles and ratiosright triangles, including the Pythagorean theorem and special triangles; perimeter and area of a triangle, including Heron's formula; thorough coverage of bisectors, medians, and altitudes, including the incenter, circumcenter, centroid, and orthocenter (though the concepts of inscribed or circumscribed circles are reserved for Volume 2); the triangle inequality; quadrilaterals; and polygons. The author, Chris McMullen, Ph.D., has over twenty years of experience teaching math skills to physics students. He prepared this workbook of the Improve Your Math Fluency series to share his strategies for solving geometry problems and formulating proofs.