Videos on geometry. Basic understanding of Algebra I necessary. After this, you'll be ready for Trigonometry.
Geometry on Khan Academy: We are surrounded by space. And that space contains lots of things. And these things have shapes. In geometry we are concerned with the nature of these shapes, how we define them, and what they teach us about the world at largefrom math to architecture to biology to astronomy (and everything in between). Learning geometry is about more than just taking your medicine ("It's good for you!"), it's at the core of everything that existsincluding you. Having said all that, some of the specific topics we'll cover include angles, intersecting lines, right triangles, perimeter, area, volume, circles, triangles, quadrilaterals, analytic geometry, and geometric constructions. Wow. That's a lot. To summarize: it's difficult to imagine any area of math that is more widely used than geometry.
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Lessons 

1) Euclid as the father of geometry  Introduction to Euclidean geometry  GeometryWe don't normally delve too far into history when talking about math, but sometimes it's important to have perspective about how some of our math concepts came about and how influential they have beco ...769 views

2) Basic geometry: language and labels  Introduction to Euclidean geometry  GeometryBefore learning any new concept (mathematical or otherwise), it's important we learn and use a common language and label concepts consistently.806 views

3) Intro to lines, line segments, and rays  GeometryLet's get familiar with the difference between lines, line segments, and rays. Hint: a ray is somewhere between a line and a line segment! Practice this lesson yourself on KhanAcademy.org right now: ...847 views

4) Language and notation of the circle  Introduction to Euclidean geometry  GeometryFormal definition of a circle. Tangent and secant lines. Diameters and radii. major and minor arcs764 views

5) Angle basics  Angles and intersecting lines  GeometryThe naming of an angle is related to the line segments that create it. We'll also learn about the angle's vertex. Come on. Let's do this.846 views

6) Measuring angles in degrees  Angles and intersecting lines  GeometryLet's come up with an exact way to measure an angle using a protractor. We'll also learn about acute angles. Not to be confused with augly angles :)748 views

7) Measuring angles using a protractor  Angles and intersecting lines  GeometryLet's use our virtual protractor to measure some angles together, shall we?824 views

8) More angle measurements using a protractor  Angles and intersecting lines  GeometryWe're practicing measuring angles using the virtual protractor. Do this a few times with us and you'll use a real protractor like a pro! Practice this lesson yourself on KhanAcademy.org right now: ...696 views

9) Complementary and supplementary angles  Angles and intersecting lines  GeometryLet's add some tools to our geometry tool belt. Here's a thorough explanation of complementary and supplementary angles, as well as definitions of adjacent and straight angles. Practice this lesson y ...776 views

10) Introduction to vertical angles  Angles and intersecting lines  GeometryBy using our knowledge of supplementary, adjacent, and vertical angles, we can solve problems involving the intersection of two lines. Including this one! Practice this lesson yourself on KhanAcademy ...679 views

11) Proof: Vertical angles are equal  Angles and intersecting lines  GeometryProving that vertical angles are equal Practice this lesson yourself on KhanAcademy.org right now: https://www.khanacademy.org/math/geome ...709 views

12) Angles formed by parallel lines and transversals  GeometryParallel lines, transversal lines, corresponding angles Practice this lesson yourself on KhanAcademy.org right now: https://www.khanacademy.org/math/geome ...821 views

13) Proof  Sum of Measures of Angles in a Triangle are 180Proof that the sum of the measures of the angles in a triangle are 180931 views

14) Triangle angle example 1  Angles and intersecting lines  GeometryFiguring out angles in a triangle. A little about exterior angles being the sum of the remote interior angles816 views

15) Triangle angle example 2  Angles and intersecting lines  GeometryAnother example finding angles in triangles766 views

16) Triangle angle example 3  Angles and intersecting lines  GeometryMultiple ways to solve for the angles of multiple triangles825 views

17) Challenging triangle angle problem  Angles and intersecting lines  GeometryInteresting problem finding the sums of particular exterior angles of an irregular pentagon919 views

18) Proof: Corresponding angle equivalence implies parallel lines  GeometryProof by contradiction that corresponding angle equivalence implies parallel lines861 views

19) Finding more angles  Angles and intersecting lines  GeometryExample of angle hunting! Practice this lesson yourself on KhanAcademy.org right now: https://www.khanacademy.org/math/geome ...839 views

20) Sum of interior angles of a polygon  Angles and intersecting lines  GeometryShowing a generalized way to find the sum of the interior angles of any polygon Practice this lesson yourself on KhanAcademy.org right now: https://www.khanacademy.org/math/geome ...833 views

21) Congruent triangles and SSS  Congruence  GeometryWhat it means for triangles to be congruent928 views

22) SSS to show a radius is perpendicular to a chord that it bisects  GeometryMore on the difference between a theorem and axiom. Proving a cool result using SSS926 views

23) Other triangle congruence postulates  Congruence  GeometrySSS, SAS, ASA and AAS postulates for congruent triangles. Showing AAA is only good for similarity and SSA is good for neither925 views

24) Two column proof showing segments are perpendicular  Congruence  GeometryUsing triangle congruency postulates to show that two intersecting segments are perpendicular785 views

25) Sum of the exterior angles of convex polygon  GeometryMore elegant way to find the sum of the exterior angles of a convex polygon746 views

26) Finding congruent triangles  Congruence  GeometryUsing the SSS, ASA, SAS, and AAS postulates to find congruent triangles Practice this lesson yourself on KhanAcademy.org right now: https://www.khanacademy.org/math/geome ...732 views

27) More on why SSA is not a postulate  Congruence  GeometrySSA special cases including RSH712 views

28) Congruent triangle proof example  Congruence  GeometryProving that a point is the midpoint via triangle congruency698 views

29) Congruent triangle example 2  Congruence  GeometryShowing that segments have the same length693 views

30) Congruent legs and base angles of isosceles triangles  Congruence  GeometryShowing that congruent legs imply equal base angles and vice versa Practice this lesson yourself on KhanAcademy.org right now: https://www.khanacademy.org/math/geome ...855 views

31) Equilateral triangle sides and angles congruent  Congruence  GeometryShowing that all of the angles of an equilateral triangle are 60 degrees Practice this lesson yourself on KhanAcademy.org right now: https://www.khanacademy.org/math/geome ...765 views

32) Equilateral and isosceles example problems  Congruence  GeometryThree example problems involving isosceles and equilateral triangles. 2 from Art of Problem Solving (by Richard Rusczyk) book Practice this lesson yourself on KhanAcademy.org right now: https://www ...722 views

33) Another isosceles example problem  Congruence  GeometryPossible angles for an isosceles triangle691 views

34) Example involving an isosceles triangle and parallel lines  Congruence  GeometryIntegrating what we know about isosceles triangles and parallel lines643 views

35) Figuring out all the angles for congruent triangles example  Congruence  GeometryAnother example of using congruency to figure out a bunch of angles775 views

36) Perimeter and area: the basics  Perimeter, area, and volume  GeometryGreat geometry primer on perimeter and area. Listen carefully. Sal explains the concepts so you'll understand them. Practice this lesson yourself on KhanAcademy.org right now: https://www.khanacade ...689 views

37) Triangle area proofs  Perimeter, area, and volume  GeometryProving that the area of any triangle is 1/2 b x h Practice this lesson yourself on KhanAcademy.org right now: https://www.khanacademy.org/math/geome ...816 views

38) Interesting perimeter and area problems  Perimeter, area, and volume  GeometryThree example problems involving perimeter and area855 views

39) Koch snowflake fractal  Perimeter, area, and volume  GeometryA shape that has an infinite perimeter but finite area678 views

40) Area of an equilateral triangle  Perimeter, area, and volume  GeometryFinding the formula for the area of an equilateral triangle with side s643 views

41) Area of Koch snowflake (part 1)  advanced  Perimeter, area, and volume  GeometryStarting to figure out the area of a Koch Snowflake (which has an infinite perimeter)716 views

42) Area of Koch snowflake (part 2)  advanced  Perimeter, area, and volume  GeometrySumming an infinite geometric series to finally find the finite area of a Koch Snowflake617 views

43) Challenging perimeter problem  Perimeter, area, and volume  GeometryPerimeter of rectangle covered by 9 nonoverlapping squares. From 2000 American Invitational Math Exam773 views

44) Similar triangle basics  Similarity  GeometryIntroduction to what it means for triangles to be similar Practice this lesson yourself on KhanAcademy.org right now: https://www.khanacademy.org/math/geome ...710 views

45) Similarity postulates  Similarity  GeometryThinking about what we need to know to determine whether two triangles are similar Practice this lesson yourself on KhanAcademy.org right now: https://www.khanacademy.org/math/geome ...593 views

46) Similar triangle example problems  Similarity  GeometryMultiple examples looking for similarity of triangles Practice this lesson yourself on KhanAcademy.org right now: https://www.khanacademy.org/math/geome ...856 views

47) Similarity example problems  Similarity  GeometryTwo example problems involving similarity Practice this lesson yourself on KhanAcademy.org right now: https://www.khanacademy.org/math/geome ...734 views

48) Challenging similarity problem  Similarity  GeometryInteresting similarity problem where we don't have a lot of information to work with Practice this lesson yourself on KhanAcademy.org right now: https://www.khanacademy.org/math/geome ...648 views

49) Similarity example where same side plays different roles  Similarity  GeometryThe same side not corresponding to itself in two similar triangles Practice this lesson yourself on KhanAcademy.org right now: https://www.khanacademy.org/math/geome ...637 views

50) Finding area using similarity and congruence  Similarity  GeometryExample of using similarity and congruence to find the area of a triangle Practice this lesson yourself on KhanAcademy.org right now: https://www.khanacademy.org/math/geome ...683 views

51) Pythagorean theorem proof using similarity  GeometryProof of the Pythagorean Theorem using similarity Practice this lesson yourself on KhanAcademy.org right now: https://www.khanacademy.org/math/geome ...683 views

52) 306090 triangle side ratios proof  Right triangles and trigonometry  GeometryProving the ratios between the sides of a 306090 triangle819 views

53) 454590 triangle side ratios  Right triangles and trigonometry  GeometryShowing the ratios of the sides of a 454590 triangle are 1:1:sqrt(2)739 views

54) 306090 triangle example problem  Right triangles and trigonometry  GeometryUsing what we know about 306090 triangles to solve what at first seems to be a challenging problem Practice this lesson yourself on KhanAcademy.org right now: https://www.khanacademy.org/math/g ...844 views

55) The golden ratio  Introduction to Euclidean geometry  GeometryAn introduction to one of the most amazing ideas/numbers in mathematics721 views

56) Introduction to angles (old)  Angles and intersecting lines  GeometryWhat an angle is. Angles in a circle. Complementary and supplementary angles.773 views

57) Angles (part 2)  Angles and intersecting lines  GeometryMore on complementary and supplementary angles. Introduction to opposite angles.754 views

58) Angles (part 3)  Angles and intersecting lines  GeometryAngles formed when a transversal intersects parallel lines.802 views

59) Angles formed between transversals and parallel lines  GeometryAngles of parallel lines730 views

60) Angles of parallel lines 2  Angles and intersecting lines  GeometryAngles of parallel lines examples540 views

61) The angle game  Angles and intersecting lines  GeometryUsing what we know to solve for angles in the Angle Game.722 views

62) Similar triangles  Similarity  GeometryIntroduction to similar triangles644 views

63) Similar triangles (part 2)  Similarity  GeometryMore on similar triangles854 views

64) Angle game (part 2)  Angles and intersecting lines  GeometryMore examples of the Angle Game.774 views

65) Acute right and obtuse angles  Angles and intersecting lines  GeometryWe briefly discussed the types of angles in the last tutorial but we'll delve a little deeper into acute, obtuse, and right angles here. Practice this lesson yourself on KhanAcademy.org right now: ...716 views

66) Area and PerimeterArea of rectangles and triangles. Perimeter of rectangles.741 views

67) Circles: radius, diameter, circumference and Pi  GeometryA circle is at the foundation of geometry and how its parts relate to each other is both completely logical and a wonder. Practice this lesson yourself on KhanAcademy.org right now: https://www.k ...691 views

68) Area of a circleArea of a circle and how it relates to radius and diameter746 views

69) The Pythagorean theorem intro  Right triangles and trigonometry  GeometryIntroduction to the Pythagorean Theorem Practice this lesson yourself on KhanAcademy.org right now: https://www.khanacademy.org/math/geome ...775 views

70) Pythagorean theorem II  Right triangles and trigonometry  GeometryMore Pythagorean Theorem examples. Introduction to 454590 triangles.692 views

71) 454590 triangles  Right triangles and trigonometry  GeometryIntroduction to 454590 Triangles780 views

72) Intro to 306090 triangles  Right triangles and trigonometry  GeometryA few more 454590 examples and an introduction to 306090 triangles.590 views

73) 306090 Triangles IIMore examples using 306090 triangles. More free lessons at: http://www.khanacademy.org/video?v=3mLUJ671 views

74) Find the volume of a triangular prism and cube  GeometryIn this tutorial we'll practice finding solid geometry volume. Keep paper and pencil nearby in case you want to remember the formulas. Practice this lesson yourself on KhanAcademy.org right now: ...719 views

75) Cylinder volume and surface area  Perimeter, area, and volume  GeometryFinding the volume and surface area of a cylinder Practice this lesson yourself on KhanAcademy.org right now: https://www.khanacademy.org/math/geome ...786 views

76) Heron's formula  Perimeter, area, and volume  GeometryUsing Heron's Formula to determine the area of a triangle while only knowing the lengths of the sides686 views

77) Part 1 of proof of Heron's formula  Perimeter, area, and volume  GeometryPart 1 of the proof of Heron's Formula696 views

78) Part 2 of proof of Heron's formula  Perimeter, area, and volume  GeometryShowing that the expression in part 1 is identical to Heron's Formula758 views

79) Inscribed and central angles  Circles  GeometryShowing that an inscribed angle is half of a central angle that subtends the same arc Practice this lesson yourself on KhanAcademy.org right now: https://www.khanacademy.org/math/geome ...904 views

80) Area of inscribed equilateral triangle (some basic trig used)  Circles  GeometryProblem that requires us to figure out the area of an equilateral triangle inscribed in a circle (A little trigonometry used)609 views

81) Right Triangles Inscribed in Circles (Proof)Proof showing that a triangle inscribed in a circle having a diameter as one side is a right triangle839 views

82) Area of diagonal generated triangles of rectangle are equal  GeometryArea of Diagonal Generated Triangles of Rectangle are Equal649 views

83) Triangle medians and centroids  Special properties and parts of triangles  GeometrySeeing that the centroid is 2/3 of the way along every median622 views

84) Triangle medians and centroids (2D proof)  GeometryShowing that the centroid is 2/3 of the way along a median664 views

85) Rhombus diagonals  Quadrilaterals  GeometryProof that the diagonals of a rhombus are perpendicular bisectors of each other721 views

87) Perpendicular radius bisects chord  Congruence  GeometrySimple proof using RSH postulate to show that a radius perpendicular to a chord bisects it792 views

88) Circumcenter of a triangle  Special properties and parts of triangles  GeometryMultiple proofs showing that a point is on a perpendicular bisector of a segment if and only if it is equidistant from the endpoints. Using this to establish the circumcenter, circumradius, and circum ...644 views

89) Circumcenter of a right triangle  GeometryShowing that the midpoint of the hypotenuse is the circumcenter711 views

90) Three points defining a circle  Special properties and parts of triangles  GeometryThree points uniquely define a circle. The center of a circle is the circumcenter of any triangle the circle is circumscribed about.661 views

91) Pointline distance and angle bisectors  GeometryThinking about the distance between a point and a line. Proof that a point on an angle bisector is equidistant to the sides of the angle and a point equidistant to the sides is on an angle bisector553 views

92) Incenter and incircles of a triangle  GeometryUsing angle bisectors to find the incenter and incircle of a triangle579 views

93) Angle bisector theorem proof  Special properties and parts of triangles  GeometryWhat the angle bisector theorem is and its proof855 views

94) Inradius, perimeter, and area  Special properties and parts of triangles  GeometryShowing that area is equal to inradius times semiperimeter690 views

95) Angle bisector theorem examples  GeometryUsing the angle bisector theorem to solve for sides of a triangle Practice this lesson yourself on KhanAcademy.org right now: https://www.khanacademy.org/math/geome ...1,060 views

96) Medians divide into smaller triangles of equal area  GeometryShowing that the three medians of a triangle divide it into six smaller triangles of equal area. Brief discussion of the centroid as well635 views

97) Exploring medial triangles  Special properties and parts of triangles  GeometryWhat a medial triangle is and its properties717 views

98) Proving that the centroid is 23rds along the median  GeometryShowing that the centroid divides each median into segments with a 2:1 ratio (or that the centroid is 2/3 along the median)616 views

99) Median centroid right triangle example  GeometryExample involving properties of medians908 views

100) Proof: Triangle altitudes are concurrent (orthocenter)  GeometryShowing that any triangle can be the medial triangle for some larger triangle. Using this to show that the altitudes of a triangle are concurrent (at the orthocenter).614 views

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