问题
填空题
设Sn=
|
答案
∵Sn=
+1 1×4
+…+1 4×7 1 (3n-2)(3n+1)
=
(1-1 3
+1 4
-1 4
+…+1 7
-1 3n-2
)1 3n+1
=
(1-1 3
)1 3n+1
∴s10=
(1-1 3
)=1 31 10 31
故答案为：10 31
搜题请搜索小程序“爱下问搜题”
设Sn=
|
∵Sn=
+1 1×4
+…+1 4×7 1 (3n-2)(3n+1)
=
(1-1 3
+1 4
-1 4
+…+1 7
-1 3n-2
)1 3n+1
=
(1-1 3
)1 3n+1
∴s10=
(1-1 3
)=1 31 10 31
故答案为：10 31