<span class="equation-text" data-index="0" data-equation="a_n=a_1" contenteditable="false"><span></span><span></span></span>+(n-1)d=<span class="equation-text" data-index="1" data-equation="a_{m}" contenteditable="false"><span></span><span></span></span>+(n-m)d=pn+q（p=q，q=<span class="equation-text" contenteditable="false" data-index="2" data-equation="a_1"><span></span><span></span></span>-d）
