Chapter 3 - Linear and Quadratic Functions - 3.2 Building Linear Models from Data - 3.2 Assess Your Understanding - Page 133: 1

Not a function

We are given the points: $(1,5),(2,6),(3,9),(1,12)$ Plot the points. The relation $\{(1,5),(2,6),(3,9),(1,12)\}$ is not a function because there are two pairs in which the first element is the same, while the second element is different: $(1,5)$ and $(1,12)$