Proving Collinearity A Step-by-Step Guide

How to prove 3 points are collinear


Collinearity is a fundamental concept in geometry that refers to points lying on the same straight line. Proving that three points are collinear requires careful analysis and application of specific techniques. In this article, we will explore various methods to establish collinearity, enabling you to confidently demonstrate the alignment of three given points.

Using Slopes

One of the most common approaches to proving collinearity involves analyzing the slopes of lines formed by the given points. Let’s assume we have three points: A(x1, y1), B(x2, y2), and C(x3, y3).

  • Calculate the slopes Determine the slopes of lines AB and BC using the slope formula: m = (y2 – y1) / (x2 – x1). If the slopes of AB and BC are equal, the points are potentially collinear.
  • Verify collinearity To confirm collinearity, calculate the slope of AC. If the slope of AC is equal to the slopes of AB and BC, the three points lie on the same line.
  • Exceptional case Watch out for vertical lines. If the slope formula results in division by zero, it implies that the points are aligned vertically. In this case, check if the x-coordinates of all three points are the same. If so, they are collinear.

Using Area Calculation

Another method to establish collinearity is by calculating the areas formed by the three points A, B, and C.

  • Form triangles Create triangles by connecting any two points among A, B, and C.
  • Calculate areas Compute the areas of triangles ABC, BCA, and CAB using the shoelace formula or other suitable techniques. If the areas are zero or have the same sign, the points are collinear.
  • Explanation When the points are collinear, the area of the triangle formed by them will be zero. This result occurs because the base of the triangle is a straight line, resulting in a height of zero. If the points are not collinear, the areas will differ or have opposite signs.

Using Vector Analysis

Vector analysis provides another effective method for proving collinearity. Here’s how it can be done

  • Form vectors Create vectors AB and AC using the given points A, B, and C.
  • Calculate cross product Compute the cross product of vectors AB and AC. If the cross product is zero, the vectors are parallel and, therefore, collinear.
  • Explanation The cross product between two vectors measures their perpendicularity. When the vectors are parallel, the cross product will be zero, indicating that the points lie on the same line.

Frequently Asked Questions

How do you prove 3 points are collinear Grade 8?

Three or more points are said to be collinear if they all lie on the same straight line. If A, B and C are collinear then m A B = m B C ( = m A C ) .

How do you prove 3 points are collinear in 3d Class 12?

As we know, a triangle cannot be formed using collinear points. So to check the Collinearity of 3 points, the area of the triangle should be equal to zero. Let P(x1,y1,z1), Q (x2,y2,z2), and R(x3,y3,z3) be any three points.


Proving collinearity requires careful analysis and understanding of various techniques such as slope comparison, area calculation, and vector analysis.

In conclusion, understanding these methods empowers you to confidently establish the collinearity of three given points, enabling you to tackle geometric problems with ease. By employing the techniques discussed, you can effectively prove whether three points lie on a straight line.

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