We first introduce methods of finding the sum of the first n terms for an arithmetic sequence of higher order.
我们首先介绍了高阶等差数列前 n 项和的求解方法。
You have n branches on the tree and you count the number of leaves and sum them up.
你计算了树上n个树枝，并数出树叶的数目并计算出总和。
N ow, work in groups to sum up the names of the clothes we have learnt and each group asks one student to report the results to the class.
学生进行小组讨论，归纳出所学过的服装名称。然后推荐一位代表向全班学生汇报，看哪一组归纳得最多。
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