Math CentralQuandaries & Queries


Question from Patrick, a parent:

On page 17 of my son's 9th grade math book Geometry by Jurgensen, Brown, and Jurgensen (copyright 2004),
the authors state from the diagram that you can conclude the points A, B, and C are collinear. In the diagram, the points A, B, and C appear to be situated on a straight line; however, nothing in the diagram states that it is a straight line. I told him that you can't conclude they are collinear because angle ABC could be 179.9 degrees instead of 180 degrees since nothing in the diagram states otherwise. His teacher marked his answer incorrect on a test but I still believe that I am correct. How do conclude points are collinear if nothing tells you that they are situated on a line?


It's difficult to give a definitive response without seeing the page and the instructions. If the three points have no other relationship then being on the page and seeming to fall on a straight line then I agree that you can't conclude in a mathematical sense that they are collinear. The teacher and the textbook authors however at this point might just be looking for an understanding of the term collinear.


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