问题
填空题
求和：
|
答案
设Sn=
+1 1×4
+…+1 4×7 1 (3n-2)×(3n+1)
则3Sn=
+3 1×4
+…+3 4×7
=1-3 (3n-2)×(3n+1)
+1 4
-1 4
+…+1 7
-1 (3n-2)
=1-1 (3n+1)
=1 (3n+1) 3n (3n+1)
所以Sn=
．n (3n+1)
故答案为n (3n+1)
搜题请搜索小程序“爱下问搜题”