console
function jis(){
var a=parseInt(document.getElementById("a").value);
var b=parseInt(document.getElementById("b").value);
var sel =document.getElementById("select").value;
var result='';
switch(sel)
{
case '+': result= a+b ;break;
case '-': result= a-b ;break;
}
document.getElementById("d").value=result;
}
<body>
<div class="top" style="height:70px;border-bottom:1px solid #ddd;">
<img class="logoIndex" height="60" src="https://www.zenitour.com/img/logoIndex.png" style="margin-left:10px;margin-top:5px;">
</div>
<div class="content" style="text-align:center;margin-top:50px;height: 78%;">
<p>简单的加减法工具</p>
<input id="a" type="text" class="sty"/>
<select id="select">
<option >+</option>
<option >-</option>
</select>
<input id="b" type="text" class="sty"/>
<button type="button" onclick="jis()">=</button>
<input id="d" type="text" class="sty" />
</div>
<div id="bottom" style="text-align:center;border-top:1px solid #ddd;">
<span class="centerspan" style="font-size:10px">Copyright © 2017 Zenitour lnc All Rights Reserved 版权所有-哲途教育 渝ICP备13002955号-2</span>
</body>
html,body {
width: 100%;
height: 100%;
margin: 0;
padding: 0;
}