为什么不加speed参数时没有效果，speed不是可选参数么
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<title>使用fadeTo()方法设置淡入淡出效果的不透明度</title>
<script src="http://libs.baidu.com/jquery/1.9.0/jquery.js" type="text/javascript"></script>
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<h3>使用fadeTo()方法设置淡入淡出效果的不透明度</h3>
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<script type="text/javascript">
$(function () {
$("span").each(function (index) {
switch (index) {
case 0:
$(this).fadeTo(0.2);
break;
case 1:
$(this).fadeTo(1000,0.4);
break;
case 2:
$(this).fadeTo(1000,0.6);
break;
}
});
});
</script>
</body>
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