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【简答题】
某校高一学生共有500人，为了了解学生的历史学习情况，随机抽取了50名学生，对他们一年来4 次考试的历史平均成绩进行统计，得到频率分布直方图如图所示，后三组频数成等比数列。 （1）求第五、六组的频数，补全频率分布直方图； （2）若每组数据用该组区间中点值（例如区间[70，80）的中点值是75）作为代表，试估计该校高一学生历史成绩的平均分； （3）估计该校高一学生历史成绩在70～100分范围内的人数。
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