求和：1+
|
an=
=1 1+2+3+…+n
，2 n(n+1)
∴Sn=a1+a2+a3+…+an
=2（
+1 1×2
+1 2×3
+…+1 3×4
）1 n×(n+1)
=2×(1-
+1 2
-1 2
+1 3
-1 3
+…+1 4
-1 n
)1 n+1
=2（1-
）=1 n+1
．2n n+1
故答案：
．2n n+1
| 完形填空。 | ||||
| It was a Saturday afternoon. Mr. Green was reading a newspaper. Suddenly he heard his wife 1 in the next room. He hurried to her and asked, "What happened to you. my dear?" "Oh, dear!" his wife cried out, "My head hurts! 2 a doctor. Hurry up!" Their two children were both 3 at that moment and something was wrong with the telephone. Mr. Green 4 go to a hospital himself. Bad luck! All the hospitals in the town were 5 that day. Mr. Green didn't know 6 . Just then his friend, Mr. Brown, saw him and asked."What is 7 ?" Mr. Green told him everything. The man.said."Why not turn to 8 ? He can help you." Mr. Green remembered his friend, Mr. Black was a good doctor. He hurried to Mr. Black's. When he got there, Mr. and Mr. Black were having their 9 meal. The doctor asked Mr. Green to drink with him. He was happy and sat 10 . After that they began to chat. And when Mrs. Black asked, "How is you wife?" "She is fine, thanks…" Mr. Green stopped at once - he remembered his wife was waiting for a doctor at home. | ||||
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