Which relation is a function?
(Points : 4)
{(1, 2); (2, 3); (3, 4); (4, 5)}
{(1, 2); (1, 3); (1, 4); (1, 5)}
{(1, 2); (2, 3); (3, 4); (2, 5)}
{(1, 2); (3, 2); (3, 3); (4, 2)}

Question

Which relation is a function?
(Points : 4)
       {(1, 2); (2, 3); (3, 4); (4, 5)}
       {(1, 2); (1, 3); (1, 4); (1, 5)}
       {(1, 2); (2, 3); (3, 4); (2, 5)}
       {(1, 2); (3, 2); (3, 3); (4, 2)}

mathmate · Answer

Here's  what you can use to tell what's NOT a function.
"A function cannot have two points each with the same x-value but different y-values".

Examples:

{(2,3),(2,4)}
is not a function.

{(1,2),(2,3),(3,4)} 
is a function.

Points (1,2),(3,4),(1,2)  make a function.