Here is an intuitive attempt at showing you how come a 1/3 is in the formula of a Cone. I do make the assumption that you are familiar with the formula of a pyramid!
Proof of Pyramid (general full proof):
https://youtu.be/F952z0McAhw
Proof of Pyramid (intuition for specific case):
https://youtu.be/J5_QXAYWHxs
Desmos.com Code used in the video:
https://www.desmos.com/calculator/gfyruypbkv
If this video helps one person, then it has served its purpose!
#help1inspire1M
Entire High School Math Video series:
1mjourney.com (Click on 1Mjourney - Math)