GRE数学必知的最基本概念
<p> 对<span word="gre">gre</span>数学最基本概念都不清楚的话，相信成绩一定不会很高。<span word="gre">gre</span>数学考试最重要的其实就是要把基础打好，题目不是很难，只要会运用这些基本概念一定能取得很好的成绩。</p><p> 1、<span word="mode">mode</span>一堆数中出现频率最高的一个或几个数，例如：<span word="mode">mode</span> <span word="of">of</span> 1,1,1,2,3,0,0,0,5 <span word="is">is</span> 1 <span word="and">and</span> 0.</p><p> 2、<span word="range">range</span></p><p> 一堆数中最大和最小数之差，例如：<span word="range">range</span> <span word="of">of</span> 1,1,2,3,5 <span word="is">is</span> 5-1=4</p><p> 3、<span word="mean">mean</span></p><p> <span word="arithmatic">arithmatic</span> <span word="mean">mean</span>，<span word="geometric">geometric</span> <span word="mean">mean</span></p><p> 4、<span word="median">median</span></p><p> 将一堆数排序之后，正中间的一个数，或者中间两个数的平均数，例如：<span word="median">median</span> <span word="of">of</span> 1,7,4,9,2,2,2,2,2,5,8 <span word="is">is</span> 2，<span word="median">median</span> <span word="of">of</span> 1,7,4,9,2,5 <span word="is">is</span> /2=6。</p><p> 5、<span word="standard">standard</span> <span word="error">error</span></p><p> 一堆数中，每个数与平均数的差的绝对值之和，除以这堆数的个数。例如：</p><p> <span word="standard">standard</span> <span word="error">error</span> <span word="of">of</span> 0,2,5,7,6 <span word="is">is</span>: /5=2.4</p><p> 6、<span word="standard">standard</span> <span word="variation">variation</span>一堆数中，每个数与平均数之差的平方之和，再除以<span word="n">n</span>，例如：<span word="standard">standard</span> <span word="variation">variation</span> <span word="of">of</span> 0,2,5,7,6 <span word="is">is</span>6.8</p><p> 7、<span word="standard">standard</span> <span word="deviation">deviation</span>就是<span word="standard">standard</span> <span word="variation">variation</span>的平方根，标准方差的公式：<span word="d">d</span>^2=/<span word="n">n</span>，<span word="d">d</span> 为标准方差。</p><p> 8. 三角形余玄定理<span word="C">C</span>^2=<span word="A">A</span>^2+<span word="B">B</span>^2-2<span word="ABCOSt">ABCOSt</span>，其中<span word="t">t</span>为<span word="AB">AB</span>两条线间的夹角。</p><p> 9. <span word="Y">Y</span>=<span word="k">k</span>1<span word="X">X</span>+<span word="B">B</span>1,<span word="Y">Y</span>=<span word="k">k</span>2<span word="X">X</span>+<span word="B">B</span>2，两线垂直的条件为<span word="K">K</span>1<span word="K">K</span>2=-1</p>
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