1 V V N2U U N N N N3 1, 1, N N1THÌ (1) 4V U G2U
3 , 1 v v n
2
u u n n n N3 1, 1,
n
n
1
thì (1) 4v u g2u .1
1
Suy ra : vn
= 3n – 1
.v1
= 4.3n – 1
. 0.5đ Vậy : un
= 4.3n – 1
– 12n2
– 12n – 1 = 4.3n – 1
– 12(n2
+ n + 2) . n nTa có +) 1 + 2 + … + n = ( 1)n n nn +) 12
22
...2
( 1)(2 1)6 +) 4(30
+3
1
+ 32
+….+
3
n – 1
)n
4 3 1 = 3 1 Thay n= 2017