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

Solution:

Using the definition of factorial, n! = n (n - 1) (n - 2) .... 3 · 2 · 1

We will use this in each of the given problems.

(i) n = 6, r = 2
6!/(6 − 2)! = 6!/(4)! = 6×5×4!/(4!) = 6×5 = 30

(ii) n = 9, r = 5
9!/(9 − 5)! = 9!/(4)! = 9×8×7×6×5×4!/(4!) = 9×8×7×6×5 = 15,120

NCERT Solutions Class 11 Maths Chapter 7 Exercise 7.2 Question 5

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

Summary:

(i) When n = 6, r = 2, n!/(n− r)! = 30 (ii) When n = 9, r = 5, n!/(n− r)! = 15,120