Use the vertical line test and select the graphs below in which represent y as a function of x.

Graphs 1 and 3 are functions.

Step-by-step explanation:

A curve in the xy-plane is a function if and only if no vertical line intersects the curve more than once. The Vertical Line Test allows us to know whether or not a graph is actually a function. Remember that a function can only take on one output for each input.

In the attached image, I've conducted the Vertical Line Test to demonstrate which are the graphs that represent y as a function of x.

The 1st and 3rd graphs (left-hand side graphs) represent a function because each vertical line drawn crosses those curves at most once.

The 2nd and 4th graph do not represent a function because, at most x-values, a vertical line would intersect the graph at more than one point.

check the picture below.

well, the angle at the vertex d is clearly not a right-angle, let's check for the one at the vertex f which does look like,   however if it's indeed, that means ef is perpendicular to df, in which case their slopes will be negative reciprocal of each other.

so the negative reciprocal of the slope of ef is 1, however the slope of df is not 1, is 1/2, so nope, they're not perpendicular lines.