A²ₙ = n! / (n-2)! = (n-2)!(n-1)n / (n-2)! = (n-1)n = n² -n
C²ₙ₊₁ = (n+1)!/ 2!*(n+1-2)! = (n+1)! /(n-1)!*2 = (n-1)!n(n+1) / (n-1)!*2 = (n²+n)/2
=> (n² +n)/2 = [2(n²-n) + n ]/3
3(n²+n) = 2(2n²-2n +n)
3n²+n = 4n² -2n
3n² -4n² +n+2n = 0
-n² +3n = 0 /*(-1)
n² - 3n = 0
n(n-3) = 0
n= 0 sau n= 3
