Question 1203313
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The relation x+y < 3 is not a function because an input like x = 2 corresponds to infinitely many y outputs.
x+y < 3
2+y < 3
y < 3-2
y < 1
Simply select any value smaller than 1
Therefore points like (2,0), (2,-1), (2, -2), etc are all solutions. 
They form a vertical column. 
As such, this example is a visual indication we have failed the vertical line test.


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If on the other hand the relation is x+y = 3, then this would be a function. Each x input corresponds to one and exactly one y output. The graph of x+y = 3, aka y = -x+3, passes the vertical line test.
{{{graph(400,400,-5,5,-5,5,-100,-x+3)}}}
This diagonal line passes through (0,3) and (3,0). These points represent the y-intercept and x-intercept respectively.


The green graph passes the vertical line test. This is because it is impossible to draw a single vertical line through more than one point on the green line.


With regard to the function y = -x+3, we have:
Domain = set of all real numbers
Range = set of all real numbers
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