Geometry

Learn Geometry

Lines, triangles, and coordinates: shapes you can see and reason about.

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Free to start · adaptive placement finds your level · reviews timed to your own forgetting.

49 lessons in Geometry

Angles in a triangleParallel lines & transversalsSquare roots & radicalsThe Pythagorean theoremSpecial right trianglesDistance & coordinatesSimilar trianglesPolygons & area formulasComposite & shaded-region areaCircles & inscribed anglesArc length & sector areaChord, secant & tangent propertiesSolids: volume & surface areaSurface area of solidsCross-sections of solidsTransformationsTriangle centersClassical proofsCoordinate-geometry proofsThe law of cosinesThe law of sinesSolving triangles & the ambiguous caseVectors in geometryConic sections as lociGolden ratio & FibonacciCompass & straightedge constructionsEuler's polyhedron formula & Platonic solidsNon-Euclidean geometry (preview)Angle chasing & Langley's adventitious anglesPower of a point & the radical axisCeva's & Menelaus's theoremsMass points & barycentric coordinatesStewart's theorem & cevian lengthsPtolemy's theorem & cyclic quadrilateralsBrahmagupta's formula & Morley's miracleIncircle, excircles & the contact triangleThe nine-point circle & Euler lineEuler's theorem & the OI distanceDirected angles & the incenter/excenter lemmaThe Simson lineInversion in a circleSpiral similarity & the Miquel pointIsogonal conjugates & symmediansPole, polar & harmonic divisionProjective geometry introClassifying plane isometriesWallpaper & frieze symmetry groupsSpherical geometryHyperbolic geometry

Built to be understood — and remembered.

Every idea is taught with motivation and a worked example before the drills, and an FSRS spaced-repetition engine schedules each review for the day its model predicts you would forget it. A short placement check finds what you already know, so you start Geometry exactly where it's useful.

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