Undefined slopeUndefined slope is so often misunderstood and many students find it confusing that I thought it would be a good thing to dedicate a special lesson to explain it. All you do is moving straight up or straight down only. You are not moving horizontally at all The slope is therefore at its steepest. A good real life example of undefined slope is an elevator. It got its name "undefined" from the fact that it is impossible to divide by zero Recall that 6/2 equal 3 because 3 × 2 = 6 However, it is impossible to do 5/0 because there exist no number you can multiply 0 by to get 5 We say that this division is undefined Now, let us try to get the slope for one of the lines above, say the one in the middle The points are (1, 3) and (1, 9) Let x_{1} = 1 , y_{1} = 9 and x_{2} = 1 and y_{2} = 3 m = (y_{1} − y_{2})/((x_{1} − x_{2}) m = (9 − 3)/(1 − 1) m = 6/0 Since there exist no number you can multiply 0 by to get 6, we say that the slope is undefined Notice that the xvalues is the same for both points (x_{1} = x_{2} = 1) . This is also the case for all the lines above In general, when the xvalues or xcoordinates is the same for both points, the slope is undefined If you can make the above observation, there is no need to compute the slope 




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