jquery --练习1（导航收缩效果）
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<span>第一章认识jQuery</span>
<a>1-1</a>
<a>1-2</a>
<a>1-3</a>
<a>1-4</a>
<a>1-5</a>
<a>1-6</a>
<a>1-7</a>
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<span>第二章认识jQuery</span>
<a>2-1</a>
<a>2-2</a>
<a>2-3</a>
<a>2-4</a>
<a>2-5</a>
<a>2-6</a>
<a>2-7</a>
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<div class="has_children">
<span>第三章认识jQuery</span>
<a>3-1</a>
<a>3-2</a>
<a>3-3</a>
<a>3-4</a>
<a>3-5</a>
<a>3-6</a>
<a>3-7</a>
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