数列：-11×2，12×3，-13×4，14×5，…的一个通项公式为______

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问题填空题

数列：-

1×2

，

2×3

，-

3×4

，

4×5

，…的一个通项公式为______．

答案

观察数列可知分母为以项数与项数加1的乘积的形式的数列，分母是常数1的数列，各项的符号正负相间，

故可得数列的通项公式a_n=

(-1)n

n(n+1)

（n∈Z^*），

故答案为：

(-1)n

n(n+1)

．

选择题

They _____ on the program for almost one week before I joined them, and now, we ____on it as no good results have come out so far.

A．had been working; are still working

B．had worked; were still working

C．have been working; have worked

D．have worked; are still working

填空题

听和说是（）得基础，是儿童入学后顺利过渡的必要条件。