Geometry
Triangles
Identifying types of triangles, using principles of similarity and congruence, and applying and proving triangle postulates.
  1. Proof - Sum of Measures of Angles in a Triangle are 180
  2. Triangle Angle Example 1
  3. Triangle Angle Example 2
  4. Triangle Angle Example 3
  5. Challenging Triangle Angle Problem
  6. Congruent and Similar Triangles
  7. Congruent Triangles and SSS
  8. SSS to Show a Radius is Perpendicular to a Chord that it Bisects
  9. Other Triangle Congruence Postulates
  10. Finding Congruent Triangles
  11. More on why SSA is not a postulate
  12. Congruent Triangle Proof Example
  13. Congruent Triangle Example 2
  14. Congruent legs and base angles of Isosceles Triangles
  15. Equilateral Triangle Sides and Angles Congruent
  16. Equilateral and Isosceles Example Problems
  17. Another Isosceles Example Problem
  18. Example involving an isosceles triangle and parallel lines
  19. Figuring out all the angles for congruent triangles example
  20. Triangle Area Proofs
  21. Area of an Equilateral Triangle
  22. Koch Snowflake Fractal
  23. Area of Koch Snowflake (part 1) - Advanced
  24. Area of Koch Snowflake (part 2) - Advanced
  25. Similar Triangle Basics
  26. Similarity Postulates
  27. Similar triangles
  28. Similar triangles (part 2)
  29. Similar Triangle Example Problems
  30. Similarity Example Problems
  31. Application of Similar Triangles
  32. Challenging Similarity Problem
  33. Similarity example where same side plays different roles
  34. Finding Area Using Similarity and Congruence
  35. Introduction to the Pythagorean Theorem
  36. The Pythagorean Theorem
  37. Pythagorean Theorem II
  1. Pythagorean Theorem 1
  2. Pythagorean Theorem 2
  3. Pythagorean Theorem 3
  4. Garfield's proof of the Pythagorean Theorem
  5. Bhaskara's proof of Pythagorean Theorem
  6. Pythagorean Theorem Proof Using Similarity
  7. Another Pythagorean Theorem Proof
  8. 45-45-90 Triangles
  9. 45-45-90 Triangle Side Ratios
  10. Intro to 30-60-90 Triangles
  11. 30-60-90 Triangles II
  12. 30-60-90 Triangle Side Ratios Proof
  13. 30-60-90 Triangle Example Problem
  14. Heron's Formula
  15. Part 1 of Proof of Heron's Formula
  16. Part 2 of the Proof of Heron's Formula
  17. Area of Inscribed Equilateral Triangle (some basic trig used)
  18. Right Triangles Inscribed in Circles (Proof)
  19. Area of Diagonal Generated Triangles of Rectangle are Equal
  20. Circumcenter of a Triangle
  21. Circumcenter of a Right Triangle
  22. Incenter and incircles of a triangle
  23. Medians divide into smaller triangles of equal area
  24. Exploring Medial Triangles
  25. Proving that the Centroid is 2-3rds along the Median
  26. Median Centroid Right Triangle Example
  27. Proof - Triangle Altitudes are Concurrent (Orthocenter)
  28. Review of Triangle Properties
  29. Euler Line
  30. Euler's Line Proof
  31. Common Orthocenter and Centroid
  32. Basic Triangle Proofs Module Example
  33. Basic Triangle Proofs Module Example 2
  34. Fill-in-the-blank triangle proofs example 1
  35. Wrong statements in proofs example 1
  36. Fill-in-the-blank triangle proofs example 2