If a vertical line intersects the graph at more than one point then it is not a function.
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A graph is a function if any vertical line intersects it at most once.
In simple terms, if a vertical line meets a graph in more than one place it is not a function.
They mean the graph/function is decreasing.
The Vertical Line Test An example might be x=cos(y). At any value of x between -1 a nd +1 (a vertical line on the graph) this is multivalued (and so it is called "multivalued"). The relation is a function, because given y you can calculate x. x is a function of y. The relation between y and x can also be written y=cos-1(x) "y is the angle whose cosine is x". From that point of view you can say " y is not a function of x" because for each x, there is more than one y that satisfyies the equation. To summarize, in this example x is a function of y but y is not a function of x.
A graph is a representation of a thing/system, and can be used to test a hypothesis. For example, if you have a graph of a trend you can find the function of that trend. Then, you can plug in values the graph defines--say, at 2 the graph reaches 5--and if the function works, you know you have modeled the phenomenon correctly. This function testing can work to test a hypothesis, especially in finding trends.