Conditions of Collinearity of Three Points

We will discuss here how to prove the conditions of collinearity of three points.

Collinear points: Three points A, B and C are said to be collinear if they lie on the same straight line.

There points A, B and C will be collinear if AB + BC = AC as is clear from the adjoining figure.

In general, three points A, B and C are collinear if the sum of the lengths of any two line segments among AB, BC and CA is equal to the length of the remaining line segment, that is,

either AB + BC = AC or AC +CB = AB or BA + AC = BC.

In other words,

There points A, B and C are collinear iff:

(i) AB + BC = AC i.e.,

Or, (ii) AB + AC = BC i.e. ,

Or, AC + BC = AB i.e.,

Solved examples to prove the collinearity of three points:

1. Prove that the points A (1, 1), B (-2, 7) and (3, -3) are collinear.

Solution:

Let A (1, 1), B (-2, 7) and C (3, -3) be the given points. Then,

AB = \(\sqrt{(-2 - 1)^{2} + (7 - 1)^{2}}\) = \(\sqrt{(-3)^{2} + 6^{2}}\) = \(\sqrt{9 + 36}\) = \(\sqrt{45}\) = 3\(\sqrt{5}\) units.

BC = \(\sqrt{(3 + 2)^{2} + (-3 - 7)^{2}}\) = \(\sqrt{5^{2} + (-10)^{2}}\) = \(\sqrt{25 + 100}\) = \(\sqrt{125}\) = 5\(\sqrt{5}\) units.

AC = \(\sqrt{(3 - 1)^{2} + (-3 - 1)^{2}}\) = \(\sqrt{2^{2} + (-4)^{2}}\) = \(\sqrt{4 + 16}\) = \(\sqrt{20}\) = 2\(\sqrt{5}\) units.

Therefore, AB + AC = 3\(\sqrt{5}\) + 2\(\sqrt{5}\) units = 5\(\sqrt{5}\) = BC

Thus, AB + AC = BC

Hence, the given points A, B, C are collinear.

 

2. Use the distance formula to show the points (1, -1), (6, 4) and (4, 2) are collinear.

Solution:

Let the points be A (1, -1), B (6, 4) and C (4, 2). Then,

AB = \(\sqrt{(6 - 1)^{2} + (4 + 1)^{2}}\) = \(\sqrt{5^{2} + 5^{2}}\) = \(\sqrt{25 + 25}\) = \(\sqrt{50}\) = 5\(\sqrt{2}\)

BC = \(\sqrt{(4 - 6)^{2} + (2 - 4)^{2}}\) = \(\sqrt{(-2)^{2} + (-2)^{2}}\) = \(\sqrt{4 + 4}\) = \(\sqrt{8}\) = 2\(\sqrt{2}\)

and

AC = \(\sqrt{(4 - 1)^{2} + (2 + 1)^{2}}\) = \(\sqrt{3^{2} + 3^{2}}\) = \(\sqrt{9 + 9}\) = \(\sqrt{18}\) = 3\(\sqrt{2}\)

⟹ BC + AC = 2\(\sqrt{2}\) + 3\(\sqrt{2}\) = 5\(\sqrt{2}\) = AB

So, the points A, B and C are collinear with C lying between A and B.

 

3. Use the distance formula to show the points (2, 3), (8, 11) and (-1, -1) are collinear.

Solution:

Let the points be A (2, 3), B (8, 11) and C (-1, -1). Then,

AB = \(\sqrt{(2 - 8)^{2} + (3 - 11)^{2}}\) = \(\sqrt{6^{2} + (-8)^{2}}\) = \(\sqrt{36 + 64}\) = \(\sqrt{100}\) = 10

BC = \(\sqrt{(8 - (-1))^{2} + (11 - (-1))^{2}}\) = \(\sqrt{9^{2} + 12^{2}}\) = \(\sqrt{81 + 144}\) = \(\sqrt{225}\) = 15

and

CA = \(\sqrt{((-1) - 2)^{2} + ((-1) + 3)^{2}}\) = \(\sqrt{(-3)^{2} + (-4)^{2}}\) = \(\sqrt{9 + 16}\) = \(\sqrt{25}\) = 5

⟹ AB + CA = 10 + 5 = 15 = BC

Hence, the given points A, B, C are collinear.

 Distance and Section Formulae




10th Grade Math

From Conditions of Collinearity of Three Points to HOME PAGE


New! Comments

Have your say about what you just read! Leave me a comment in the box below. Ask a Question or Answer a Question.



Didn't find what you were looking for? Or want to know more information about Math Only Math. Use this Google Search to find what you need.



Share this page: What’s this?

Recent Articles

  1. Adding 5-digit Numbers with Regrouping | 5-digit Addition |Addition

    Mar 18, 24 02:31 PM

    Adding 5-digit Numbers with Regrouping
    We will learn adding 5-digit numbers with regrouping. We have learnt the addition of 4-digit numbers with regrouping and now in the same way we will do addition of 5-digit numbers with regrouping. We…

    Read More

  2. Adding 4-digit Numbers with Regrouping | 4-digit Addition |Addition

    Mar 18, 24 12:19 PM

    Adding 4-digit Numbers with Regrouping
    We will learn adding 4-digit numbers with regrouping. Addition of 4-digit numbers can be done in the same way as we do addition of smaller numbers. We first arrange the numbers one below the other in…

    Read More

  3. Worksheet on Adding 4-digit Numbers without Regrouping | Answers |Math

    Mar 16, 24 05:02 PM

    Missing Digits in Addition
    In worksheet on adding 4-digit numbers without regrouping we will solve the addition of 4-digit numbers without regrouping or without carrying, 4-digit vertical addition, arrange in columns and add an…

    Read More

  4. Adding 4-digit Numbers without Regrouping | 4-digit Addition |Addition

    Mar 15, 24 04:52 PM

    Adding 4-digit Numbers without Regrouping
    We will learn adding 4-digit numbers without regrouping. We first arrange the numbers one below the other in place value columns and then add the digits under each column as shown in the following exa…

    Read More

  5. Addition of Three 3-Digit Numbers | With and With out Regrouping |Math

    Mar 15, 24 04:33 PM

    Addition of Three 3-Digit Numbers Without Regrouping
    Without regrouping: Adding three 3-digit numbers is same as adding two 3-digit numbers.

    Read More