用Javascript实现一个基本的运算器，具体内容如下
使用表格布局，JS添加事件
<td class="tdd" colspan="4"></td>
</tr>
<tr class="trr1">
<td class="tdd1" colspan="2"><input class="btn1 btn" type="button" value="c"/></td>
<td class="tdd1" colspan="2"><input class="btn1 btn" type="button" value="d"/></td>
</tr>
<tr class="trr">
<td class="tdd"><input class="btn" type="button" value="7"/></td>
<td class="tdd"><input class="btn" type="button" value="8"/></td>
<td class="tdd"><input class="btn" type="button" value="9"/></td>
<td class="tdd"><input class="btn" type="button" value="/"/></td>
</tr>
<tr class="trr">
<td class="tdd"><input class="btn" type="button" value="4"/></td>
<td class="tdd"><input class="btn" type="button" value="5"/></td>
<td class="tdd"><input class="btn" type="button" value="6"/></td>
<td class="tdd"><input class="btn" type="button" value="*"/></td>
</tr>
<tr class="trr">
<td class="tdd"><input class="btn" type="button" value="1"/></td>
<td class="tdd"><input class="btn" type="button" value="2"/></td>
<td class="tdd"><input class="btn" type="button" value="3"/></td>
<td class="tdd"><input class="btn" type="button" value="-"/></td>
</tr>
<tr class="trr">
<td class="tdd"><input class="btn" type="button" value="0"/></td>
<td class="tdd"><input class="btn" type="button" value="."/></td>
<td class="tdd"><input class="btn" type="button" value="+"/></td>
<td class="tdd"><input class="btn" type="button" value="="/></td>
</tr>
</table>
