Question 1204093
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Answers
Slope of vertical line = <font color=red size=4>undefined</font>
Slope of horizontal line = <font color=red size=4>0</font>



Reason


Consider, for example, the two points (5,7) and (5,9).
They are on a vertical line.
Plug those coordinates into the slope formula {{{m = (y[2]-y[1])/(x[2]-x[1])}}}
Afterward you'll find the denominator turns to 0. This is a problem. 
We cannot have 0 in the denominator. This leads to "undefined".



Now consider the two points (1,2) and (5,2) which are on a horizontal line.
Plug those coordinates into the slope formula mentioned earlier, and you'll get a slope of 0. This time the numerator is 0 which is allowed. 
0/x = 0 when x is nonzero.
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