Why slicing a cone gives an ellipse

Dandelin spheres, conic sections, and a view of genius in math. Brought to you by you: Home page: Thoughts on the recent change to be sponsor-free: Video on Feynman’s lost lecture: I originally saw the proof of this video when I was reading Paul Lockhart’s “Measurement“, which I highly recommend to all math learners, young and old. New shirts/mugs available: The 3d animations in the video were done using Grapher, while 2d animations were done using If you want to contribute translated subtitles or to help review those that have already been made by others and need approval, you can click the gear icon in the video and go to subtitles/cc, then “add subtitles/cc“. I really appreciate those who do this, as it helps make the lessons accessible to more people. Music by Vincent Rubinetti: