Question 5 Evaluate, when
(i) n = 6, r = 2
(ii) n = 9, r = 5
Class X1 - Maths -Permutations and Combinations Page 141
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n!/(n - r)! = ?
when, (1) n = 6 and r = 2
6!/( 6 - 2)!
= 6!/4!
= 6 × 5 × 4!/4! [ we know, n! = n(n-1)(n-2)...1 ]
= 6 × 5
= 30
(ii) when n = 9 and r = 5
9!/( 9 - 5)!
= 9!/4!
= 9 × 8 × 7 × 6 × 5 × 4!/4!
= 9 × 8 × 7 × 6 × 5
= 15120
when, (1) n = 6 and r = 2
6!/( 6 - 2)!
= 6!/4!
= 6 × 5 × 4!/4! [ we know, n! = n(n-1)(n-2)...1 ]
= 6 × 5
= 30
(ii) when n = 9 and r = 5
9!/( 9 - 5)!
= 9!/4!
= 9 × 8 × 7 × 6 × 5 × 4!/4!
= 9 × 8 × 7 × 6 × 5
= 15120
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