s-selector-tag">d</span>, <span class="hljs-selector-tag">a</span> ∦ <span class="hljs-selector-tag">d</span> 
求证: <span class="hljs-selector-tag">b</span> ∦ <span class="hljs-selector-tag">c</span>
∵ <span class="hljs-selector-tag">a</span> ∥ <span class="hljs-selector-tag">b</span>
   <span class="hljs-selector-tag">a</span> ∦ <span class="hljs-selector-tag">d</span>
∴ <span class="hljs-selector-tag">b</span> ∦ <span class="hljs-selector-tag">d</span>
∵ <span class="hljs-selector-tag">