Need help finding "n" in Permutations!

mr fantastic

MHF Hall of Fame
Dec 2007
16,948
6,768
Zeitgeist
Link 1: http://i42.tinypic.com/zu2fwj.png
Link 2: http://i40.tinypic.com/e8rhvs.png

Please help me. I do not know how to solve for n. Thank you.
Link1: \(\displaystyle ^nP_8 = 13 \cdot ^nP_7\).

Substitute the definition \(\displaystyle ^nP_r\) and simplify. Then solve the resulting simple linear equation for n. Note that (n - 7)! = (n - 7)(n - 8)!


Link2: \(\displaystyle ^nP_6 = 90 \cdot ^nP_4\).

Done in a similar way. Yiou will end up with a quadratic equation to solve. Note that one of the solutions to this quadratic equation is not be valid.
 
  • Like
Reactions: krzyrice
Jan 2008
155
13
Pangkor Island, Perak, Malaysia.
  • Like
Reactions: krzyrice