Geometry
The Common Core high-school Geometry sequence, transformations, congruence, similarity, circles, and solids, as animations you watch happen.
13 units · 103 topics · 467 questions
The undefined terms and basic objects points, lines, angles that every later definition and proof in the course is built from.
Points, Lines, and Planes · Segments, Length, and Betweenness · Rays and Measuring Angles · Classifying Angles by Measure · +5 more
What a transversal does to two parallel lines, and how those angle patterns build toward every triangle and polygon angle sum.
Parallel and Perpendicular Lines · Classifying Triangles by Sides and Angles · Angles Formed by a Transversal · Proving Two Lines Are Parallel · +3 more
Sliding, flipping, and turning a shape without changing its size, and the surprising thing that happens when you chain two moves together.
Translations · Reflections · Rotations · Composing Two Reflections · +3 more
The shortcuts that prove two triangles match without checking all six parts, why one shortcut secretly fails, and how to write the argument down.
Congruent Triangles and CPCTC · SSS Congruence · SAS Congruence · ASA and AAS Congruence · +6 more
Four special points hiding inside every triangle, built from bisectors, medians, and altitudes, and the surprising line that connects three of them.
Perpendicular Bisectors and the Circumcenter · Angle Bisectors and the Incenter · Medians and the Centroid · Altitudes and the Orthocenter · +3 more
Scaling a shape up or down without distorting it, the shortcuts for proving two triangles are similar, and what scaling secretly does to area.
Dilations and Scale Factor · Similar Polygons · Triangle Similarity Criteria: AA, SAS, SSS · Parallel Lines Cutting Proportional Sides · +3 more
The Pythagorean theorem proved a dozen different ways, the two special right triangles worth memorizing, and the ratios that unlock any right triangle.
The Pythagorean Theorem · Multiple Proofs of the Pythagorean Theorem · Special Right Triangles: 45-45-90 and 30-60-90 · Sine, Cosine, and Tangent Ratios · +6 more
Parallelograms, rectangles, rhombuses, and kites sorted into a family tree by which properties they do and don't share.
Properties of Parallelograms · Proving a Quadrilateral Is a Parallelogram · Rectangles, Rhombuses, and Squares · Trapezoids and Kites · +1 more
Turning a shape's dimensions into the number of square units it covers, then using that same area to measure the odds of landing on a target.
Area and Perimeter of Triangles and Parallelograms · Area of Trapezoids, Kites, and Rhombuses · Area of Regular Polygons · Area of Composite Figures · +3 more
Every angle a circle can make, from its center, from its edge, and from outside it entirely, plus the arcs and sectors those angles cut off.
Circumference and Pi · Parts of a Circle · Central Angles and Arcs · Inscribed Angles · +7 more
Points, lines, and circles rewritten as equations, and how a proof can be run entirely with algebra instead of a compass.
The Distance Formula · The Midpoint Formula · Slope, Parallel Lines, and Perpendicular Lines · Distance from a Point to a Line · +3 more
Wrapping and filling three-dimensional shapes, the slices hiding inside them, and the spinning trick that turns a flat shape solid.
Nets and Surface Area of Prisms and Cylinders · Surface Area of Pyramids and Cones · Volume of Prisms and Cylinders · Volume of Pyramids and Cones · +6 more
Every classic compass-and-straightedge construction, drawn step by step, and why the arcs alone are enough to guarantee exactness.
Constructing Congruent Segments and Angles · Constructing a Perpendicular Bisector · Constructing an Angle Bisector · Constructing Parallel and Perpendicular Lines · +2 more