# Geometry

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Learn about shapes and their properties such as size and position. Created by Khan Academy.

Recommended that you have a good foundation in Algebra before taking this course.

###### Average Course Length

20 hours

###### Skill Level

Intermediate

##### Pick a lesson

1: Euclid as the Father of Geometry

2: Language and Notation of Basic Geometry

3: Lines, Line Segments, and Rays

4: Language and Notation of the Circle

5: Angle Basics

6: Measuring Angles in Degrees

7: Using a Protractor

8: Measuring Angles

9: Complementary and Supplementary Angles

10: Angles at the intersection of two lines

11: Proof-Vertical Angles are Equal

12: Angles Formed by Parallel Lines and Transversals

13: Proof - Sum of Measures of Angles in a Triangle are 180

14: Triangle Angle Example 1

15: Triangle Angle Example 2

16: Triangle Angle Example 3

17: Challenging Triangle Angle Problem

18: Proof - Corresponding Angle Equivalence Implies Parallel Lines

19: Finding more angles

20: Sum of Interior Angles of a Polygon

21: Congruent Triangles and SSS

22: SSS to Show a Radius is Perpendicular to a Chord that it Bisects

23: Other Triangle Congruence Postulates

24: Two column proof showing segments are perpendicular

25: Sum of the exterior angles of convex polygon

26: Finding Congruent Triangles

27: More on why SSA is not a postulate

28: Congruent Triangle Proof Example

29: Congruent Triangle Example 2

30: Congruent legs and base angles of Isosceles Triangles

31: Equilateral Triangle Sides and Angles Congruent

32: Equilateral and Isosceles Example Problems

33: Another Isosceles Example Problem

34: Example involving an isosceles triangle and parallel lines

35: Figuring out all the angles for congruent triangles example

36: Perimeter and Area Basics

37: Triangle Area Proofs

38: Interesting Perimeter and Area Problems

39: Koch Snowflake Fractal

40: Area of an Equilateral Triangle

41: Area of Koch Snowflake (part 1) - Advanced

42: Area of Koch Snowflake (part 2) - Advanced

43: Challenging Perimeter Problem

44: Similar Triangle Basics

45: Similarity Postulates

46: Similar Triangle Example Problems

47: Similarity Example Problems

48: Challenging Similarity Problem

49: Similarity example where same side plays different roles

50: Finding Area Using Similarity and Congruence

51: Pythagorean Theorem Proof Using Similarity

52: 30-60-90 Triangle Side Ratios Proof

53: 45-45-90 Triangle Side Ratios

54: 30-60-90 Triangle Example Problem

55: The Golden Ratio

56: Introduction to angles

57: Angles (part 2)

58: Angles (part 3)

59: Angles formed between transversals and parallel lines

60: Angles of parallel lines 2

61: The Angle Game

62: Similar triangles

63: Similar triangles (part 2)

64: Angle Game (part 2)

65: Acute Right and Obtuse Angles

66: Area and Perimeter

67: Circles: Radius, Diameter and Circumference

68: Area of a circle

69: The Pythagorean Theorem

70: Pythagorean Theorem II

71: 45-45-90 Triangles

72: Intro to 30-60-90 Triangles

73: 30-60-90 Triangles II

74: Solid Geometry Volume

75: Cylinder Volume and Surface Area

76: Heron's Formula

77: Part 1 of Proof of Heron's Formula

78: Part 2 of the Proof of Heron's Formula

79: Inscribed and Central Angles

80: Area of Inscribed Equilateral Triangle (some basic trig used)

81: Right Triangles Inscribed in Circles (Proof)

82: Area of Diagonal Generated Triangles of Rectangle are Equal

83: Triangle Medians and Centroids

84: Triangle Medians and Centroids (2D Proof)

85: Rhombus Diagonals

86: 2003 AIME II Problem 7

87: Perpendicular Radius Bisects Chord

88: Circumcenter of a Triangle

89: Circumcenter of a Right Triangle

90: Three Points Defining a Circle

91: Point Line Distance and Angle Bisectors

92: Incenter and incircles of a triangle

93: Angle Bisector Theorem Proof

94: Inradius Perimeter and Area

95: Angle Bisector Theorem Examples

96: Medians divide into smaller triangles of equal area

97: Exploring Medial Triangles

98: Proving that the Centroid is 2-3rds along the Median

99: Median Centroid Right Triangle Example

100: Proof - Triangle Altitudes are Concurrent (Orthocenter)

101: Review of Triangle Properties

102: Euler Line

103: Euler's Line Proof

104: Common Orthocenter and Centroid

105: Quadrilateral Overview

106: Proof - Opposite Sides of Parallelogram Congruent

107: Proof - Diagonals of a Parallelogram Bisect Each Other

108: Proof - Opposite Angles of Parallelogram Congruent

109: Proof - Rhombus Diagonals are Perpendicular Bisectors

110: Proof - Rhombus Area Half Product of Diagonal Length

111: Area of a Parallelogram

112: Problem involving angle derived from square and circle

113: Area of a Regular Hexagon