Question 83510
Find the slope of the line passing through the points (2,-5) and (-10,-5)
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Your line has a slope = [-5--5]/[-10-2]= 0/-12 = 0
Notice that the two points have the same y value so the line
is parallel to the x axis.  The slope is zero because y does
not change no matter how much change there is in x.
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Undefined refers to a slope whose value cannot be stated.
A vertical line (like x=5) has an "undefined" slope because the 
y-value rises any amount you care to consider even though
the x value does not change; so the slope = (any number)/0
which is "undefined".
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Cheers,
Stan H.