GeoGebra
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GeoGebra's free browser-based geometry app for constructing points, lines, triangles, polygons, and circles. Measurements update live as figures are dragged, letting learners test conjectures about angles, congruence, and polygon properties and see why classical constructions work.
More resources on Triangles & Polygons
Introduction to Geometry
Explore triangles, polygons, and geometric fundamentals in this introductory course from AoPS Instructors! Perfect for building a strong foundation.
Geometry
Master triangles and polygons with Sal Khan's Geometry course! Learn shapes, angles, and spatial reasoning.
Geometry: A Guided Inquiry
A high school geometry textbook built around guided problems that ask students to discover results themselves. Develops congruence, similarity, area, polygons, circles, and proof through cooperative inquiry, helping readers reason about triangles and polygons rather than memorize theorems.
mathsisfun.com
MathIsFun.com is a beginner-friendly math site that explains concepts with clear, step-by-step lessons and interactive activities. It covers topics from algebra and linear equations to geometry, fractions, data, and more, with practice problems, worksheets, and quizzes.
Plane Geometry
Classic American school textbook, reprinted by Forgotten Books, covering rectilinear figures, congruent triangles, parallel lines, polygons, circles, proportion, similar figures and areas through numbered theorems and graded exercises. Working through it builds fluency in writing rigorous Euclidean proofs about triangles and polygons.