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<head>
<script src="https://libs.baidu.com/jquery/1.10.2/jquery.min.js">
</script>
<script>
$(document).ready(function(){
$("p:nth-child(3n+2)").css("background-color","yellow");
});
</script>
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<body>
<p>The p:nth-child(3n+2) selects each 3rd paragraph, starting at the 2nd child element.
</p><div style="border:1px solid;">
<span>This is a span element</span>
<p>The first paragraph (and the second child in div).</p>
<p>The second paragraph.</p>
<p>The third paragraph.</p>
<p>The fourth paragraph.</p>
<p>The fifth paragraph.</p>
<p>The sixth paragraph.</p>
<p>The seventh paragraph.</p>
<p>The eight paragraph.</p>
</div><br>
</body>
</html>