Solution 67: Substitution and Symmetry vs. a Functional Equation

From a channel Lumen includes in full: LetsSolveMathProblems works through competition maths problems (Putnam, olympiads) and university topics in full, with rigorous explanations.

A 12-minute video from LetsSolveMathProblems's catalogue; the title names its topic.

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