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Geometry Building Blocks

This is a series of free, online High School Geometry Video Lessons and solutions.

Videos, worksheets, solutions, and activities to help Geometry students.

In these lessons, we will learn

  • point, line and plane
  • counterexample
  • what is a good geometry definition
  • postulate, axiom and conjecture

Three Undefined Terms: Point, Line, and Plane

In Geometry, we have several undefined terms: point, line and plane. From these three undefined terms, all other terms in Geometry can be defined. In Geometry, we define a point as a location and no size. A line is defined as something that extends infinitely in either direction but has no width and is one dimensional while a plane extends infinitely in two dimensions.
This video explains and demonstrates the fundamental concepts (undefined terms) of geometry: point, line, plane.
This video explains basic geometry concepts. A visual 3 Dimentional Demonstration of points, lines, and planes


Throughout Geometry, students write definitions and test conjectures using counterexamples. When writing definitions, counterexamples are useful because they ensure a complete and unique description of a term. If a counterexample does not exist for a conjecture (an if - then statement), then the conjecture is true.
This is a demonstration of the counterexample method.
How to use a counterexample to prove a definition or conjecture incorrect.
This video gives 4 example problems explaining counterexamples to conditional statements. The first 2 examples are the basics behind counterexamples. The last 2 examples are problems you'd probably see on a quiz or test.


Good Geometry Definition

Writing a definition is a common exercise during the early stages of Geometry. An excellent geometry definition will classify, quantify, and not have a counterexample. Once a term is defined, it can be used in subsequent definitions; for example, once parallel lines are defined, they can be used in the definition of a parallelogram. How to write a good definition that does not have a counterexample.

Postulate, Axiom, Conjecture

Three words that are used seemingly interchangeably in Geometry are postulate, axiom, and conjecture. It is important, however, to know how each word is different and to know the subtle implications of using each word. These terms are especially important when working with Geometry proofs.
How to differentiate between the words postulate, axiom, and conjecture.


You can use the Mathway widget below to practice Algebra or other math topics. Try the given examples, or type in your own problem. Then click "Answer" to check your answer.

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