18. not a function
19. not a function
20. the left side graph is a function
18. Any relation to be a function, any x value should have only one y value
this is not a function because when x=7 , y= 9 and 12 (two y values for one x value)
19. x=-1 has two y values
and x=1 also has two y values
20. here, you can use the vertical line test.
the left graph passes the vertical line test which is a function because any vertical line drawn intersects the curve in only 1 point, but the right graph, the vertical line intersects the curve in 2 points, which is not a function.