((n+1)!)/((n-1)!) = 90
What is n? Learning how to solve it is my priority here.
Thanks
+130511 Whatever n+1 and n-1 are, they are two integers apart....
So......if we divided (n+1)! by (n-1)! the result = (n+1)* n
So we have
(n+1)*n = 90
n^2 + n = 90 subtract 90 from both sides
n^2 + n - 90 = 0 factor
(n -9) (n +10) = 0 so, either n - 9 = 0 or n + 10 = 0
And, since were assuming a positive integer, n = 9
So
(9 +1) ! / (9 - 1) ! =
10! / 8! =
10 * 9 =
90
