Posted by **lisa** on Wednesday, November 18, 2009 at 7:41pm.

This was the question: Explain why the formula for determining slope using the coordinates of two points does not apply to vertical lines.

The formula for determining a slope using two points does not apply to vertical lines simply because dividing by 0 is not allowed…undefined. To find a slope of a horizontal line we use the formula m= y1-y2/x1-x2: this specifies that we have two points to work with.

Vertical lines are in the form of x=some number, such as x=6; When a line involves an undefined slope the line is vertical, and when a line is vertical you will end up dividing by 0 if you try and evaluate the slope using points (-4,5) and (8,-5):

m= (5)-(5)/ (8)-(-5) =0/-13; undefined

All the points on a vertical line hold the same x coordinate, so x1= x2 and the denominator of the slope formula is zero. For that reason, the slope is undefined because division by zero is not allowed.

Vertical lines have undefined slopes. Thus, the concept of slope simply does not work for vertical lines. A slope of 0=horizontal,

According to page 530 of the text” any two points on a vertical line have the same x coordinate, thus the change in x is always 0, always undefined.

Is this right? If so, do you have any suggestions to make it less wordy and redundant.

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