题目

已知 + + +…+ ＝ ，求n的值. 答案：解：∵ 1nn+1+(n+1)n＝ nn+1+(n+1)nn2(n+1)−(n+1)2n＝ nn+1+(n+1)n−n(n+1)＝ nn ﹣ n+1n+1∴ 112+21 + 123+32 + 134+43 +…+ 1nn+1+(n+1)n ＝ 11 ﹣ 22 + 22 ﹣ 33 +……+ nn ﹣ n+1n+1 ＝1﹣ n+1n+1∴1﹣ n+1n+1 ＝ 4950 ， ∴n＝2499-We' re going for a picnic next Sunday, and I'd like you to join us. ---________! I have to see my grandmother. She has been ill for two weeks.A. What a pity!B. Good luck!C. That's all right!D. Go ahead!