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Khan Academy is a 501(c)(3) nonprofit organization with the mission of providing a free, world-class education for anyone, anywhere. Our interactive practice problems, articles, and videos help students succeed in math, biology, chemistry, physics, history, economics, finance, grammar, and many other topics.

Course content

0h08m

Euclid as the father of geometry | Introduction to Euclidean geometry | Geometry | Khan Academy

We 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 become.

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 large--from 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 exists--including you. Having said all that, some of the specific topics we'll cover include angles, intersecting lines, right triangles, perimeter, area, volume, ci

0h12m

Basic geometry: language and labels | Introduction to Euclidean geometry | Geometry | Khan Academy

Before learning any new concept (mathematical or otherwise), it's important we learn and use a common language and label concepts consistently.

Missed the previous lesson?

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 large--from 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 exists--including you. Having said all that, some of the specifi

0h03m

Intro to lines, line segments, and rays | Geometry | Khan Academy

Let's get familiar with the difference between lines, line segments, and rays. Hint: a ray is somewhere between a line and a line segment!

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Missed the previous lesson?

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 large--from math to architecture to biology to astronomy

0h11m

Language and notation of the circle | Introduction to Euclidean geometry | Geometry | Khan Academy

Formal definition of a circle. Tangent and secant lines. Diameters and radii. major and minor arcs

Missed the previous lesson?

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 large--from 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 exists--including you. Having said all that, some of the specific topics we'll cover include angles, intersecting lines, righ

0h06m

Angle basics | Angles and intersecting lines | Geometry | Khan Academy

The 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.

Missed the previous lesson?

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 large--from 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 exists--including you. Having said all that, some of th

0h08m

Measuring angles in degrees | Angles and intersecting lines | Geometry | Khan Academy

Let'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 :)

Missed the previous lesson?

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 large--from 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 exists--including you.

0h03m

Measuring angles using a protractor | Angles and intersecting lines | Geometry | Khan Academy

Let's use our virtual protractor to measure some angles together, shall we?

Missed the previous lesson?

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 large--from 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 exists--including you. Having said all that, some of the specific topics we'll cove

0h04m

More angle measurements using a protractor | Angles and intersecting lines | Geometry | Khan Academy

We'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!

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Missed the previous lesson?

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 large-

0h08m

Complementary and supplementary angles | Angles and intersecting lines | Geometry | Khan Academy

Let'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.

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Geometry on Khan Academy: We are surrounded by space. And that space contains lots of things. And these things

0h07m

Introduction to vertical angles | Angles and intersecting lines | Geometry | Khan Academy

By using our knowledge of supplementary, adjacent, and vertical angles, we can solve problems involving the intersection of two lines, including this one!

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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

0h04m

Proof: Vertical angles are equal | Angles and intersecting lines | Geometry | Khan Academy

Proving that vertical angles are equal

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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 large--from math to architecture to biology to astronomy (a

0h07m

Angles formed by parallel lines and transversals | Geometry | Khan Academy

Parallel lines, transversal lines, corresponding angles

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Missed the previous lesson?

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 large--from math to architecture to biology to astronomy (and everyt

0h06m

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

Proof that the sum of the measures of the angles in a triangle are 180

0h06m

Triangle angle example 1 | Angles and intersecting lines | Geometry | Khan Academy

Figuring out angles in a triangle. A little about exterior angles being the sum of the remote interior angles

Missed the previous lesson?

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 large--from 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 exists--i

0h04m

Triangle angle example 2 | Angles and intersecting lines | Geometry | Khan Academy

Another example finding angles in triangles

Missed the previous lesson?

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 large--from 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 exists--including you. Having said all that, some of the specific topics we'll cover include angles, int

0h05m

Triangle angle example 3 | Angles and intersecting lines | Geometry | Khan Academy

Multiple ways to solve for the angles of multiple triangles

Missed the previous lesson?

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 large--from 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 exists--including you. Having said all that, some of the specific topics we'll

0h08m

Challenging triangle angle problem | Angles and intersecting lines | Geometry | Khan Academy

Interesting problem finding the sums of particular exterior angles of an irregular pentagon

Missed the previous lesson?

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 large--from 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 exists--including yo

0h05m

Proof: Corresponding angle equivalence implies parallel lines | Geometry | Khan Academy

Proof by contradiction that corresponding angle equivalence implies parallel lines

Missed the previous lesson?

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 large--from 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 exists--including you. Having said all that, some of the spe

0h06m

Finding more angles | Angles and intersecting lines | Geometry | Khan Academy

Example of angle hunting!

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Missed the previous lesson?

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 large--from math to architecture to biology to astronomy (and everyt

0h09m

Sum of interior angles of a polygon | Angles and intersecting lines | Geometry | Khan Academy

Showing a generalized way to find the sum of the interior angles of any polygon

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