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 so it stays learned.

What you'll learn

42 lessons in Geometry

Angles in a triangleSquare roots & radicalsThe Pythagorean theoremDistance & coordinatesSimilar trianglesCircles & inscribed anglesPolygons & area formulasSolids: volume & surface areaParallel lines & transversalsSpecial right trianglesTriangle centersTransformationsChord, secant & tangent propertiesArc length & sector areaCross-sections of solidsGolden ratio & FibonacciClassical proofsCoordinate-geometry proofsProjective geometry introNon-Euclidean geometry (preview)Compass & straightedge constructionsPower of a point & the radical axisThe nine-point circle & Euler lineInversion in a circlePole, polar & harmonic divisionCeva's & Menelaus's theoremsPtolemy's theorem & cyclic quadrilateralsStewart's theorem & cevian lengthsMass points & barycentric coordinatesIncircle, excircles & the contact triangleSpiral similarity & the Miquel pointIsogonal conjugates & symmediansEuler's polyhedron formula & Platonic solidsLaw of sines & law of cosinesSolving triangles & the ambiguous caseVectors in geometryConic sections as lociSpherical geometryHyperbolic geometryClassifying plane isometriesWallpaper & frieze symmetry groupsAngle chasing & Langley's adventitious angles
How Erudia teaches

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 moment just before you'd forget it. A short placement check finds what you already know, so you start Geometry exactly where it's useful.

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