Q. 34.3( 3 Votes )

# Show that the three points A(2, 3, 4), B(-1, 2, -3) and C(-4, 1, -10) are collinear and find the ratio in which C divides AB.

Answer :

**Given:** A(2, 3, 4), B(-1, 2, -3) and C(-4, 1, -10)

**To prove:** A, B and C are collinear

**To find:** the ratio in which C divides AB

**Formula used:**

**Section Formula:**

A line AB is divided by C in m:n where A(x, y, z) and B(a, b, c).

The coordinates of C is given by,

Let C divides AB in ratio k: 1

Three points are collinear if the value of k is the same for x, y and z coordinates

Therefore, m = k and n = 1

A(2, 3, 4), B(-1, 2, -3) and C(-4, 1, -10)

Coordinates of C using section formula:

On comparing:

⇒ -k + 2 = -4(k + 1)

⇒ -k + 2 = -4k – 4

⇒ 4k – k = - 2 – 4

⇒ 3k = -6

⇒ k = -2

⇒ 2k + 3 = k + 1

⇒ 2k – k = 1 – 3

⇒ k = – 2

⇒ -3k + 4 = -10(k + 1)

⇒ -3k + 4 = -10k – 10

⇒ -3k + 10k = -10 – 4

⇒ 7k = -14

⇒ k = -2

The value of k is the same in all three times

Hence, **A, B and C are collinear**

As k = -2

**C divides AB externally in ratio 2:1**

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