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Note that when the line of the graph is vertical the slope is undefined.
You were given two points:
(1,4) and (r,5)
The slope is undefined. This means that the value of r must be a value that produces division by zero, which is not allowed in math.
What could be the value of r?
undefined slope = (5 - 4)/(r -1)
The only answer that makes sense is r = 1. When r is 1, we get a ZERO in the denominator of this fraction, which makes the slope undefined.
How did I get 1 for r? I set the denominator (r - 1) to zero and solved for r.
r - 1 = 0
r = 1
Got it? Is this clear?