NCERT Solutions for Class 11 Maths Chapter 7 Permutations and Combinations Exercise 7.2

If you want accurate NCERT solutions of Chapter 7 Permutations and Combinations Exercise 7.2 then you're at right place as Studyrankers have prepared detailed solutions. These will help you in revising the chapter and improve your marks in the examination. Class 11 Maths NCERT Solutions will help you if you want to solve difficult questions in no time.


1. Evaluate:
(i) 8 !
(ii) 4! - 3!

Answer

(i) 8! = 8 × 7 × 6 × 5 × 4 × 3 × 2×  1
= 40320
(ii) 4! = 4 × 3 × 2 × 1 = 24
3! = 3 × 2 × 1 = 6
4! – 3! = 24 – 6 = 18

2. Is 3! + 4!= 7!?

Answer

3! = 3 × 2× 1 = 6
4! = 4 × 3 × 2 × 1 = 24
LHS = 3! + 4! = 6 + 24 = 30
RHS = 7! = 7×  6 × 5 × 4 × 3 × 2 × 1 = 5040
LHS = RHS

3. Compute 8!/(6! × 2!)

Answer

8!/(6! × 2!) = (8× 7 × 6 × 5 × 4 × 3 × 2 × 1)/((6 × 5 × 4 × 3 × 2 × 1) × ( 2 × 1))
= (8 × 7)/(2 × 1) = 28

4. If 1/6! + 1/7!  = x/8! , find x.

Answer

1/6! + 1/7! = 1/6! + 1/7.6! = x/(8.7.6. !)
=> 1/6! (1 + 1/7) = x/(8.7.6. !)
=> 8/7 = x/8.7 => x = 64

5. Evaluate n!/(n – r) when
(i) n = 6, r = 2
(ii) n = 9, r = 5

Answer

(i) n = 6, r = 2
n!/(n – r)! = 6!/(6 – r) ! = 6!/4!
= (6 × 5 × 4 × 3× 2 × 1)/(4 × 3 × 2 × 1) = 6 × 5 = 30

(ii) n = 9, r = 5
n!/(n – r)! = 9! /(9 – 5)!  = 9! /4!
= (9 ×  8 × 7 × 6 × 5 × 4 × 3 × 2 × 1)/(4 × 3 × 2 × 1)
9 × 8 × 7 × 6 × 5 = 15120