Basic geometry Get up to speed on the core foundations of geometry. Community Questions Lines Lines, line segments, and rays are fundamental to geometry. Our mission: learn what they are and how to use them. Angles Measure angles and find missing angles in interesting geometry problems. Shapes Classify shapes and solve geometry problems using what we know of the properties of shapes. The coordinate plane We use coordinates to describe where something is. In geometry, coordinates say where points are on a grid we call the "coordinate plane". Area and perimeter Area and perimeter help us measure the size of 2D shapes. We’ll start with the area and perimeter of rectangles. From there, we’ll tackle trickier shapes, such as triangles and circles. Volume and surface area Volume and surface area help us measure the size of 3D objects. We’ll start with the volume and surface area of rectangular prisms. From there, we’ll tackle trickier objects, such as cones and spheres. The Pythagorean theorem The Pythagorean theorem describes a special relationship between the sides of a right triangle. Even the ancients knew of this relationship. In this topic, we’ll figure out how to use the Pythagorean theorem and prove why it works. Transformations, congruence, and similarity Learn about geometric transformations and how they're used to understand the meaning of "similar" and "congruent".