GRE数学基本概念+实例解析（6）
<p> 排列组合</p><p> 排列数公式和组合数公式：</p><p> 排列：</p><p> 组合：</p><p> 乘法原理：</p><p> 一件事的完成可以分成<span word="n">n</span>个步骤，完成其第一步有<span word="m">m</span>1种方法，完成第二步有<span word="m">m</span>2种方法完成第<span word="n">n</span>步有<span word="mn">mn</span>种方法，那么完成这件事一共有<span word="N">N</span>=<span word="m">m</span>1<span word="m">m</span>2<span word="mn">mn</span>种不同的方法。</p><p> 例题.<span word="If">If</span> <span word="all">all</span> <span word="of">of</span> <span word="the">the</span> <span word="room">room</span> <span word="numbers">numbers</span> <span word="in">in</span> <span word="a">a</span> <span word="certain">certain</span> <span word="hotel">hotel</span> <span word="must">must</span> <span word="be">be</span> <span word="even">even</span> <span word="numbers">numbers</span>, <span word="and">and</span> <span word="if">if</span> <span word="each">each</span> <span word="of">of</span> <span word="the">the</span> <span word="numbers">numbers</span> <span word="uses">uses</span> <span word="all">all</span> <span word="four">four</span> <span word="of">of</span> <span word="the">the</span> <span word="digits">digits</span> 3,4,5 <span word="and">and</span> 8, <span word="what">what</span> <span word="is">is</span> <span word="the">the</span> <span word="greatest">greatest</span> <span word="number">number</span> <span word="of">of</span> <span word="four">four</span>-<span word="digit">digit</span> <span word="numbers">numbers</span> <span word="that">that</span> <span word="the">the</span> <span word="hotel">hotel</span> <span word="can">can</span> <span word="have">have</span>?</p><p> 例题.<span word="What">What</span> <span word="is">is</span> <span word="the">the</span> <span word="total">total</span> <span word="number">number</span> <span word="of">of</span> <span word="different">different</span> 4-<span word="digit">digit</span> <span word="integers">integers</span> <span word="in">in</span> <span word="which">which</span> <span word="the">the</span> <span word="units">units</span> <span word="digit">digit</span> <span word="is">is</span> <span word="an">an</span> <span word="odd">odd</span> <span word="integer">integer</span> <span word="while">while</span> <span word="the">the</span> <span word="thousands">thousands</span> <span word="digit">digit</span> <span word="is">is</span> <span word="an">an</span> <span word="even">even</span> <span word="integer">integer</span>?</p><p> 例题.<span word="What">What</span> <span word="is">is</span> <span word="the">the</span> <span word="total">total</span> <span word="number">number</span> <span word="of">of</span> <span word="different">different</span> 5-<span word="digit">digit</span> <span word="numbers">numbers</span> <span word="that">that</span> <span word="contain">contain</span> <span word="all">all</span> <span word="of">of</span> <span word="the">the</span> <span word="digits">digits</span> 1,2,3,4, <span word="and">and</span> 5 <span word="and">and</span></p><p> 1) <span word="in">in</span> <span word="which">which</span> <span word="none">none</span> <span word="of">of</span> <span word="the">the</span> <span word="odd">odd</span> <span word="digits">digits</span> <span word="occur">occur</span> <span word="next">next</span> <span word="to">to</span> <span word="each">each</span> <span word="other">other</span>?</p><p> 2) <span word="in">in</span> <span word="which">which</span> <span word="none">none</span> <span word="of">of</span> <span word="the">the</span> <span word="even">even</span> <span word="digits">digits</span> <span word="occur">occur</span> <span word="next">next</span> <span word="to">to</span> <span word="each">each</span> <span word="other">other</span>?</p><p> 平时，建议大家一定要注意<span word="GRE">GRE</span>数学基本概念的加强理解，因为无论<span word="GRE">GRE</span>数学考试怎样的题目都是从最基本的概念中演变而来的。</p><p> </p><p> </p>
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