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www.cn992.com:【课件一】20.2数据的集中趋势与离散程度_图文

诗朗诵比赛 ?请七名同学来当评委 ?主持人负责收集分数,并登记在黑板 上(评委在打分时只保留一位小数) ?去掉一个最高分,去掉一个最低分, 请你算出三位选手的最后得分(结果 保留一位小数) 二 新课 1. 什么是平均数? 一般地,如果有n个数 x , x … xn 1 2 那么 x ? 1 (x ? x ? … ? x ) n 1 2 n 叫做这n个数的平均数,x 读作“x拔. 2. 平均数的简化计算公式 x ? x? ? a 思考: ?x 、x 、 a 各是什么? 特别是如何确定a的值呢? 练习1:请6位同学说出自己的身高, 请你计算出他们的平均身高 (结果保留一位小 数)。 练一练: 1.某班10名学生为支援“希望工程” 将平时积攒的零花钱捐献给贫困地区 的失学儿童,每人捐款金额如下(单位: 元): 10, 12, 13.5, 21, 40.8, 19.5, 20.8, 25, 16, 30 这 1010 名同学平均捐款多少元 ? 解 :这 名同学的平均捐款为 (10+12+13.5+21+40.8+19.5+20.8+25 +16+30) ÷10=20.86(元) 答:这10名同学平均捐款20.86元 一般地,当一组数据 边波动时, x1 , x2… xn的各个数值在某个数据 可将这一组数据同时减去一个常数a,得 ? ? x1 ? a, x2 ? ? x2 ? a,?, xn ? ? xn ? a x1 ? ? a, x2 ? x2 ? ? a,?, xn ? xn ? ?a x1 ? x1 1 x ? ( x1 ? x2 ? ? ? xn ) n 那么 因此 1 ? ? a ) ? ( x2 ? ? a ) ? ? ? ( xn ? ? a )? ? ?( x1 n 1 ? ? x2 ? ? ? ? xn ? ) ? na)? ? ?( x1 n 1 1 ? ? ? ? ( x1 ? x2 ? ? ? xn ) ? ? na n n ? x? ? a 练习2: 请你求下面这组数据的平均数: 105 103 110 98 101 100 101 114 108 106 想想:你是如何确 定a的值的。 x ? x? ? a 延伸与拓展 1、举例说明以下两式的实际意义 ( 12 ) ) v1t?v2t x1? 2 t1?2 2( 请你来做教练: 考考你:有一篇报道说,有一个身高 1.7 米的人在平均水深只有0.5米的一条 为了从甲乙两名学生中选拔一人参加 河流中淹死了,你感觉奇怪吗? 射击比赛,对他们的射击水平进行了 测验.两人在相同条件下各射靶10次, 命中的环数如下: 甲 7 8 6 8 6 5 9 10 7 4 乙 9 5 7 8 7 6 8 6 7 7 请问:你选哪一个人参加射击比赛? 小 结: 1、算术平均数,加权平均数的 概念。 2、会求一组数据的算术平均数, 加权平均数。 3、能用所学的知识解决一些实 际问题,知道数学来源于生 活,服务于生活。 作业: 课本练习

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