问题
选择题
用[x]表示不大于x的最大整数,如[3]=3,[3.1]=3.设S=
|
答案
[
]=[n(n+1)-2+[n(n+1)-1]2 n(n+1)
]=n(n+1)-2=(n-1)(n+2),1 n(n+1)
∵
=1 (n-1)(n+2)
(1 3
-1 n-1
),1 n+2
∴3S=
-1 9
+1 12
-1 10
+1 13
-1 11
+1 14
-1 12
+…+1 15
-1 48
=1 51
+1 9
+1 10
-1 11
-1 49
-1 50
,1 51
∴[20S]=[
×(20 3
+1 9
+1 10
-1 11
-1 49
-1 50
)]≈[1.6]=1.1 51
故选B.