Ex 1: Use the Vertical Line Test to Determine if a Graph Represents a Function

From a channel Lumen includes in full: Mathispower4u (James Sousa, a maths instructor) has thousands of short worked examples covering arithmetic through calculus, statistics and linear algebra, organised to match course topics.

A 3-minute video from Mathispower4u's catalogue; the title names its topic.

Video by Mathispower4u, played through YouTube's embedded player.