概念
分类用加法(要么<span class="equation-text" data-index="0" data-equation="\ldots" contenteditable="false"><span></span><span></span></span> 要么<span class="equation-text" data-index="1" data-equation="\ldots" contenteditable="false"><span></span><span></span></span>)<br>分步用乘法(即<span class="equation-text" data-index="2" data-equation="\ldots" contenteditable="false"><span></span><span></span></span>又<span class="equation-text" contenteditable="false" data-index="3" data-equation="\ldots"><span></span><span></span></span>)
有序用排列(不可互换)<br>无序用组合(可以互换)
