how can I change the output value into Mathjax style?

I have to use HTML and JavaScript to design a website that have two input fields (a & b)and one action button, when the button is clicked, the section below would show the addition operation result (c) of a + b Here's what I have written

<script>
    MathJax.Hub.Config({
            tex2jax: {
            inlineMath: [ ['$','$'], ["\\(","\\)"] ],
            displayMath: [ ['$$','$$'], ["\\[","\\]"] ]
            }});

</script>

<script> 
  var asw = null;
  var a=null;
  var b=null;
  function math(){

 if(asw==null) asw=document.getElementById("answer");
  a=document.getElementById("num1");
  b=document.getElementById("num2");
  a1 = parseInt(a.value);
  b2 = parseInt(b.value);
  c = a1+b2   
  asw.innerHTML = ("$$a+b=$$"+ c);

  MathJax.Hub.Queue(["Typeset",MathJax.Hub]);
  MathJax.Hub.processSectionDelay=0;

  };
</script>

<body>
<input type="button" onclick="math()" value="action">
a=<input type="text"id="num1"><br> 
b=<input type="text"id="num2">
<div id="answer" >

</div>

But in this case, if i input a=1, b=2, then the section will show only the addition result 3 in the normal way but not in the Mathjax; if I change asw.innerHTML = ("$$a+b=$$"+ c); into asw.innerHTML = ("$$a+b=$$"+ "$$c$$") ,then the result will show "a+b=c" in the mathjax, but not "a+b=3",so I want to ask that how to fix this problem?

Solution

Use

asw.innerHTML = ("$$a+b=" + c + "$$");

to have the value of c included in the math. Use

asw.innerHTML = ("$$" + a + "+" + b + "=" + c + "$$");

get get the values of a and b inserted into the math as well.