剧场有多少座位：一个剧场设置了20 排座位，第一排有38 个座位，往后每一排都比前一排多2 个座位，这个剧场一共设置了多少个座位？

数学题解答：先算出最后一排座位有多少个？38+19×2＝76（个）。

然后可以按照梯形公式来计算座位总数，对应关系是：上底38，下底76，高20。

算式：（38+76）×20÷2＝1140（个）

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