Ex 6.3,6 - Chapter 7 Class 11 Permutations and Combinations (Important Question)

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Ex 6.3, 6 Find n if n – 1P3 : nP4 = 1 : 9. Lets first calculate n – 1P3 and nP4 separately n – 1P3 = ((𝑛−1)!)/(𝑛−1 −3)! = ((𝑛−1)!)/(𝑛−4)! = (𝑛 − 1)(𝑛 − 2)(𝑛 − 3)(𝑛 − 4)!/(𝑛−4)! = (n – 1)(n – 2)(n – 3) It is given that n – 1P3 : nP4 = 1 : 9 "n – 1P3" /"n P4" = "1" /" 9" 9 (n – 1P3) = nP4 9(n – 1)(n – 2)(n – 3) = n(n – 1)(n – 2)(n – 3) 9"(n – 1)(n – 2)(n – 3)" /"(n – 1)(n – 2)(n – 3)" = n 9 = n n = 9

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Davneet Singh has done his B.Tech from Indian Institute of Technology, Kanpur. He has been teaching from the past 14 years. He provides courses for Maths, Science and Computer Science at Teachoo