## Precalculus (6th Edition) Blitzer

The set 1 has each $x$ -coordinate paired with only one $y$ -coordinate.
Consider the two sets of ordered pairs, Set 1: $\left\{ \left( 1,5 \right),\left( 2,5 \right) \right\}$ Set 2: $\left\{ \left( 5,1 \right),\left( 5,2 \right) \right\}$ In set 1, both $x$ -coordinates are different (1 and 2), and $y$ -coordinates are the same (5). So, in set 1 only one $y$ -coordinate is 5. Thus, set 1 has each $x$ -coordinate paired with only one $y$ -coordinate. Check for set 2: Set 2: $\left\{ \left( 5,1 \right),\left( 5,2 \right) \right\}$ In set 2, both $x$ -coordinates are the same (5) and the $y$ -coordinates are different (1 and 2). So, in set 2, the $y$ -coordinates are not the same. Hence, set 1 has each $x$ -coordinate paired with only one $y$ -coordinate.