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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