# Collinear

Pronunciation: /koʊˈlɪn.i.ər/ Explain

 Click on the points and drag them to change the figure. Manipulative 1 - Collinear Points Created with GeoGebra.

If points are collinear, they are in the same line. Since two points define a line, any two points are collinear (see figure 1). If three points are collinear, a line drawn using any two of the points will contain the third (see figure 2).

If objects are non-collinear, no one line can be drawn that contains all the objects (see figure 3). Figure 1: Collinear points Figure 2: Non-collinear points

### Related Words

Collinearity: Having to do with whether or not objects are in the same line.

### How to tell if Points are Collinear Figure 3: Graph with 3 points.

In 2-dimensional analytical geometry, each point has an x-coordinate and a y-coordinate. These coordinates can be used to see if three are collinear. To see if three or more points are collinear, pick one of the points as a reference point. If the slopes of the lines defined by the reference point and each of the other points are equal, the points are collinear. Table 1 shows the steps for determining if the points in figure 3 are collinear.

StepEquation(s)Discussion
1 This algorithm starts with the equation of a line in point slope form.
2 Start by selecting any two of the points. For this demonstration, select (-1,-1) and (3,1).
3 Use the slope formula to determine the slope of the line defined by the two points.
4 Now pick one of the points already used and the third point.
5 Find the slope of the line defined by those two points.
6 Since the slopes are equal, the three points are collinear.
Table 1: How to find out if three points are collinear.

1. McAdams, David E.. All Math Words Dictionary, collinear. 2nd Classroom edition 20150108-4799968. pg 36. Life is a Story Problem LLC. January 8, 2015. Buy the book
2. Casey, John, LL.D., F.R.S.. The First Six Books of the Elements of Euclid. pg 5. Translated by Casey, John, LL.D. F.R.S.. www.archive.org. Hodges, Figgis & Co.. 1890. Last Accessed 6/25/2018. http://www.archive.org/stream/firstsixbooksofe00caseuoft#page/5/mode/1up/search/collinear. Buy the book
3. Palmer, Claude Erwin and Taylor, Daniel Pomeroy. Solid Geometry. pg 280. www.archive.org. Scott, Foresman and Company. 1918. Last Accessed 6/25/2018. http://www.archive.org/stream/solidgeometry00palmrich#page/280/mode/1up/search/collinear. Buy the book

McAdams, David E. Collinear. 4/13/2019. All Math Words Encyclopedia. Life is a Story Problem LLC. http://www.allmathwords.org/en/c/collinear.html.

### Revision History

12/21/2018: Reviewed and corrected IPA pronunication. (McAdams, David E.)