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    <title>练习</title>
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*{padding:0;margin:0;}
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        for(var i=0;i<oLi.length;i++){
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             var oSpan=a.getElementsByTagName('span')[0];
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             var par=parseFloat(oSpan.innerHTML);
             oInp[0].onclick = function () {
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                 if (oEm.innerHTML < 0) {
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                 oI.innerHTML = oEm.innerHTML * par + '元';
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                 oI.innerHTML = oEm.innerHTML * par + '元';
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<body>
<ul>
    <li>
        <input type="button" value="-">
        <em>0</em>
        <input type="button" value="+">
        <p>单价<span>12.5元</span>;合计<i>0元</i></p>
    </li>
    <li>
        <input type="button" value="-">
        <em>0</em>
        <input type="button" value="+">
        <p>单价<span>20.5元</span>;合计<i>0元</i></p>
    </li>
    <li>
        <input type="button" value="-">
        <em>0</em>
        <input type="button" value="+">
        <p>单价<span>9.5元</span>;合计<i>0元</i></p>
    </li>
    <li>
        <input type="button" value="-">
        <em>0</em>
        <input type="button" value="+">
        <p>单价<span>10.5元</span>;合计<i>0元</i></p>
    </li>
</ul>
<div>
    你购买的商品合计<b>0</b>件;共<span>0</span>元
</div>
</body>
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