The unexpectedly hard windmill question (2011 IMO, P2)

From a channel Lumen includes in full: Grant Sanderson's 3Blue1Brown: animated, proof-driven explanations of linear algebra, calculus, neural networks, probability and more; widely used alongside university courses and built on careful mathematics rather than hand-waving.

A 16-minute video from 3Blue1Brown's catalogue; the title names its topic.

Video by 3Blue1Brown, played through YouTube's embedded player.