---
title: Triangles & Polygons
description: This topic explores the properties, classifications, and theorems of triangles and multi-sided polygons. Learners will understand interior and exterior angles, perimeter, area, and congruence criteria, establishing a foundation for advanced geometric and trigonometric analysis.
category: mathematics
subcategory: geometry-and-trigonometry
difficulty: beginner, intermediate, advanced
url: /subject/triangles-and-polygons
---

# Triangles & Polygons

This topic explores the properties, classifications, and theorems of triangles and multi-sided polygons. Learners will understand interior and exterior angles, perimeter, area, and congruence criteria, establishing a foundation for advanced geometric and trigonometric analysis.

## Available Resources

2 Books • 2 Courses • 2 Websites

## Websites

### 1. GeoGebra

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.

**Difficulty:** Beginner | **Price:** Free

**Link:** https://www.geogebra.org/geometry

**Tags:** geometry, dynamic-geometry, geometric-constructions, triangles, polygons

### 2. 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.

**Difficulty:** Intermediate | **Language:** English | **Price:** Free

**Link:** https://mathsisfun.com

**Tags:** websites, mathematics-statistics, algebra

## Courses

### 1. Introduction to Geometry

Explore triangles, polygons, and geometric fundamentals in this introductory course from AoPS Instructors! Perfect for building a strong foundation.

**Difficulty:** Intermediate | **Price:** Paid

**Link:** https://artofproblemsolving.com/school/course/gintro

**Tags:** geometry, triangles, polygons, euclidean-geometry, competition-math

### 2. Geometry

Master triangles and polygons with Sal Khan's Geometry course! Learn shapes, angles, and spatial reasoning.

**Difficulty:** Intermediate | **Price:** Free

**Link:** https://www.khanacademy.org/math/geometry

**Tags:** geometry, triangles, polygons, congruence, similarity

## Books

### 1. Geometry: A Guided Inquiry

**Author:** G. D. Chakerian, Calvin D. Crabill, Sherman K. Stein

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.

**Difficulty:** Intermediate | **Language:** English | **Price:** Paid

**Link:** https://www.amazon.com/dp/0895824302?tag=edmonddante07-20

**Tags:** geometry, euclidean-geometry, triangles, polygons, geometric-proof

### 2. Plane Geometry

**Author:** George Wentworth

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.

**Difficulty:** Intermediate | **Language:** English | **Price:** Paid

**Link:** https://www.amazon.com/dp/1332818161?tag=edmonddante07-20

**Tags:** euclidean-geometry, plane-geometry, triangles, polygons, geometric-proofs

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