Last updated at May 29, 2018 by Teachoo

Transcript

Example 13 Find r, if 5 4Pr = 6 4Pr-1 Now it is given that 5 4Pr = 6 5Pr-1 5 ร 4!/(4 โ ๐)! = 6 ร 5!/(5โ (๐ โ 1))! 5 ร 4!/(4 โ ๐)! = 6 ร 5!/(5 โ ๐ + 1)! 5 ร 4!/(4 โ ๐)! = 6 ร 5!/(6 โ ๐)! (6 โ ๐)!/(4 โ ๐)! = (6 ร 5!)/(5 ร 4! ) ((6 โ ๐)(5 โ ๐)(4 โ ๐)!)/(4 โ ๐)! = (6 ร 5!)/(5 ร 4! ) (6 โ r) (5 โ r) = (6 ร 5!)/(5 ร 4! ) (6 โ r) (5 โ r) = (6 ร 5 ร 4!)/(5 ร 4! ) (6 โ r)(5 โ r) = 6 6(5 โ r) โ r(5 โ r) = 6 30 โ 6r โ 5r + r2 = 6 30 โ 11r + r2 = 6 r2 โ 11r + 30 = 6 r2 โ 11r + 30 โ 6 = 0 r2 โ 11r + 24 = 0 r โ 8r โ 3r + 24 = 0 r(r - 8) โ 3(r โ 8) = 0 (r โ 8) (r โ 3) = 0 Hence r = 3, 8.

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Example 13 Important You are here

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Chapter 7 Class 11 Permutations and Combinations

Serial order wise

About the Author

Davneet Singh

Davneet Singh is a graduate from Indian Institute of Technology, Kanpur. He has been teaching from the past 8 years. He provides courses for Maths and Science at Teachoo. You can check his NCERT Solutions from Class 6 to 12.