HOW TO SHOW THE GIVEN POINTS ARE COLLINEAR USING SECTION FORMULA

If three points are collinear, then one of the points divide the line segment joining the other two points in the ratio r : 1.

The section formula can be used only when the given three points are collinear.

Example :

Using section formula, show that the points A (7, −5), B (9, −3) and C (13, 1) are collinear.

Solution :

Section formula  =  (mx2+nx1) / (m+n),  (my2+ny1) / (m+n)

Let the point B divides the line segment joining the point A and C in the ratio k : 1

k (13) + 1 (7) / (k + 1), k (1) + 1 (-5) / (k + 1)  =  (9, -3)

(13k + 7)/(k + 1), (k - 5) / (k + 1)  =  (9, -3)

Equating the x and y coordinates

(13k + 7) / (k + 1)  =  9 

(13k + 7)  =  9(k + 1)

13k + 7 = 9k + 9

13k - 9k  =  9 - 7

4k  =  2

 k = 2/4

 k = 1/2

(k - 5) / (k + 1)  =  -3

k - 5  =  -3(k + 1)

k - 5  =  -3k - 3

k + 3k  =  -3 + 5

 4k  =  2

k  =  2/4

k  =  1/2

We get the same values for k, hence the point A, B and C are collinear.

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