javascript实现简单计算器功能急！！

<HTML>[xss_clean]functioncas(){vara=document.getElementById("first").value;varb=document.getElementById("sercond".value;if(document.kfc.sign.value=="a"){document.kfc.result.value=parseInt(a)+parseInt(b);}if(document.kfc.sign.value=="b"){document.kfc.result.value=parseInt(a)-parseInt(b);}if(document.kfc.sign.value=="c"){document.kfc.result.value=parseInt(a)*parseInt(b);}if(document.kfc.sign.value=="d"){document.kfc.result.value=parseInt(a)/parseInt(b);}}[xss_clean]<BODY>输入第一个数字+-*/<选择离子值="e">%</select>第二个数字类型=结果</form></BODY></HTML>这样document.getElementById("sign")获取所有对象而不是值。

求助:用JavaScript完成一个简单的计算器

<!--第一步：在<body>区域添加以下代码-->[xss_clean]<!--Beginfunctiondoit(){form.input.value=eval(form.input.value)}functionCos(){x=form.input.valueif(x=='')alert('错误:InputRequired');elseform.input.value=Math.cos(x);}functionSin(){x=form.input.valueif(x=='')alert('错误:InputRequired');elseform.input.value=Math.sin(x);}functionLn(){x=form.input.valueif(x=='')alert('错误：InputRequired');elseform.input.value=Math.log(x);}functionRoot(){x=form.input.valueif(x=='')alert('错误：InputRequired');elseform.input.value=Math.sqrt(x);}functionTan(){x=form.input.valueif(x=='')alert('错误:InputRequired');elseform.input.value=Math.tan(x);}functionIcos(){x=form.input.valueif(x=='')alert('错误:InputR等价');elseform.input.value=Math.acos(x);}functionIsin(){x=form.input.valueif(x=='')alert('Error:InputRequired');elseform.input.value=Math.asin(x);}functionItan(){x=form.input.valueif(x=='')alert('错误:InputRequired');elseform.input.value=Math.atan(x);}functionRound(){x=form.input.valueif(x=='')alert('错误:InputRequired');elseform.input.value=Math.round(x);}functionRan(){x=form.input.valueform.input.value=Math.random(x);}functionNeg(){x=form.input.valueif(x=='')Alert('错误：InputRequired');elsex=parseFloat(x)*-1;}functiondel(){x=form.input.valuex=(x.substring)-1}//结束-->[xss_clean]Tecool高级版计算器