Question

Which of the following relations is a function?
{ (-2, 4), (-1, 3), (5, 2), (1, 4) }
{ (1, 7), (1, 8), (2, 7), (3, 8) }
{ (2, 4), (4, 6), (4, 8), (6, 4) }
{ (5, 7), (6, 3), (8, 6), (5, 4) }

Answer 1

If the ordered pairs (points) are in a set, i.e. enclosed in { }, then you can scan for duplication of the first (x-) element of the points.

If there is no repetition of the x- values, the relation is a function.
If the y-values of the points with repeated x-values are different, the relation is NOT a function.

Examples:
{(2,3),(5,8),(9,3),(15,-20)} is a function, because 2,5,9,15 have no repetition.
{(2,4),(5,2),(3,8),(2,3)} is NOT a function, because between (2,4) and (2,3), we are unable to determine the value of f(2) (=4 or =3?).