xn = (n+1)2 +(n+2)2 + [(n + 1)(n + 2)]2 xn = n2 + 2n + 1 +n2 + 4n + 4 + [n2 + 3n + 2]2 = n4 + 6n3 + 15n2 + 18n + 9 = [n2 + 3n + 3]2