The Location of Minima and Maxima in Functions of Several Variables

From a channel Lumen includes in full: MathTheBeautiful (Pavel Grinfeld, a Drexel maths professor) teaches linear algebra, tensor calculus and calculus with a focus on geometric intuition and rigour.

A 22-minute video from MathTheBeautiful's catalogue; the title names its topic.

Video by MathTheBeautiful, played through YouTube's embedded player.