Tamil Nadu Board of Secondary EducationHSC Science Class 11

# There are 11 points in a plane. No three of these lies in the same straight line except 4 points, which are collinear. Find, the number of straight lines that can be obtained from the pairs of these - Mathematics

Sum

There are 11 points in a plane. No three of these lies in the same straight line except 4 points, which are collinear. Find, the number of straight lines that can be obtained from the pairs of these points?

#### Solution

Number of points in a plane = 11

No three of these points lie in the same straight line except 4 points.

The number of straight lines that can be obtained from the pairs of these points

Through any two points, we can draw a straight line.

∴ The number straight lines through any two points of the given 11 points = 11C

= (11!)/(2! xx (11 - 2)!)

= (11!)/(2! xx 9!)

= (11 xx 10 xx 9!)/(2! xx 9!)

= (11 xx 10)/(2 xx 1)

= 11 × 5

= 55

Given that 4 points are collinear.

The number of straight lines through any two points of these 4 points is

= 4C2

= (4!)/(2!(4 - 2)!)

= (4!)/(2! xx 2!)

= (4 xx 3xx2!)/(2! xx 2!)

= (4 xx 3)/(2 xx 1)

= 2 × 3

= 6

Since these 4 points are collinear

These 4 points contribute only one line instead of the 6 lines.

The total number of straight lines that can be drawn through 11 points on a plane with 4 of the points being collinear is

= 55 – 6 + 1

= 50

Concept: Combinations
Is there an error in this question or solution?
Chapter 4: Combinatorics and Mathematical Induction - Exercise 4.3 [Page 187]

#### APPEARS IN

Tamil Nadu Board Samacheer Kalvi Class 11th Mathematics Volume 1 and 2 Answers Guide
Chapter 4 Combinatorics and Mathematical Induction
Exercise 4.3 | Q 24. (i) | Page 187
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