Question 204991
Explain why the line x = 4 is a vertical line.
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Plot two points where x = 4 such as (4,2) and (4,6)
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Plot those two points and draw a line thru them.
You will see that you get a vertical line.
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Also, if you calculate a slope thru those points 
you will get (6-2)/(0) which is undefined.  That is
the slope of a vertical line.
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Cheers,
Stan H.