Circles: radius, diameter, circumference and Pi | Geometry | Khan Academy

A circle is at the foundation of geometry and how its parts relate to each other is both completely logical and a wonder.

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, 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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[thesaaturn]

Me and the bois whatching khan academy in quarantine

[michaelmag]

Another excellent explanation from khan academy. Explaining how the formula was originally established makes it so much easier to comprehend.

[canuseetheuglybirdie]

just want to know how he can write perfectly on desktop

[crbaade]

As a mom trying to homeschool in 2021, this is so valuable. I love how he explains it step by step.

[Sansatu]

you are literally the greatest person alive. You saved me from a F in my math class

[rainstorm.]

Omg bless u I have a circle geometry test tomorrow

[nathanielackman]

i think i did a funny,
teacher: king arthur invented the circle table.
Student: no it was sir cumference.

[kenyan]

Omg. I've been doing this for the last 5 yrs of my life and never understood and understood it by half way through th video

[christylabarbera]

I'm a 55 year old college student who has always been horrible at math just trying to pass the Praxis. This is the best explanation of Pi ever!! After all these years, I understand. Thank you!!

[sophiarodriguez]

Khan is probably the BEST teacher ever!! thanks!

[ashleyrojas]

im too dumb and i have a test tommrow that determines. my future

[ambercaesar]

The public school system has failed me greatly. It took me 24 years to fully understand something he explained perfectly in 11 minutes. Thank you Khan Academy!!

[brandonseto]

I actually understood this in 10 minutes besides my math teacher

[pariswilliams]

thank you so much, you are the best teacher ever! with that 11 Minute video, you taught me more than I would have learnt in 2 lessons!

[randomlotus]

Man, thanks. This helped, I don't know. My math teacher never explains right. No one in my class understands him. They all just learn the material on their own.

[whotheheckis_ray]

My teacher has been recently absent and we've had many substitute teachers come in which interrupted me and my classmates curriculum for math, when we got the lesson on Circles I had no idea what I was being assigned and thank you so much for this video teaching me more then school.

[hausofamelia]

Wow, this made it seem much more simpler than it seemed to be! I finally understand, so thank you. :)

[oscarb]

Though I was able to follow this very easily I can understand why others can't

[thericefarmer]

Play at 0.5 speed if you want to hear a drunk man and start rolling on the floor laughing.

[kawaiipandajoy]

it really helped me catch up when I'm absent at school coz I'm always ill