An,3 = 3 .( n - 1 )

n.(n-1).(n-2) = 3.(n-1)..........=> :(n-1)

n.(n-2) = 3

n² -2n -3 = 0

-b/c = 2/-3 = 3-1/3.-1 => S:{3, -1}

mas, em fatorial, 'n' deve ser positivo => n=3 #

Ola manoel

A(n,3) = n!/(n - 3)! = 3*(n - 1)

n*(n - 2)*(n - 1)*(n - 3)!/(n - 3)! = 3*(n - 1)

n*(n - 2) = 3

n² - 2n - 3 = 0

delta

d² = 4 + 12 = 16

d = 4

n = (2 + 4)/2 = 3 (C)

pronto