Proof: A is a Subset of B iff A intersect B Equals A | Set Theory, Subsets

Picked one by one from a mixed channel. Transcript check: both title readers certain; 3 of 5 sampled certain picks confirmed by transcript, none failed. Channel rule: Include only plainly titled lesson videos that teach a specific math concept, definition, proof or procedure, typically tagged with a subject such as '| Set Theory', '| Real Analysis', '| Graph Theory', '| Linear Algebra', '| Geometry', or '| Pre-Algebra', or titled 'How to…', 'What is…', 'Definition…', or 'Proof:…'. Exclude reaction, meme, pop-culture-hook, music and hype-titled uploads.