how to express math symbols in html code example

Example: math in html

The top-level element in MathML is <math>. Every valid MathML instance must be wrapped in <math> tags. In addition you must not nest a second <math> element in another, but you can have an arbitrary number of other child elements in it.

Examples
  
Theorem of Pythagoras

HTML5 notation
<!DOCTYPE html>
<html>
  <head>
    <title>MathML in HTML5</title>
  </head>
  <body>

  <math>
    <mrow>
      <mrow>
        <msup>
          <mi>a</mi>
          <mn>2</mn>
        </msup>
        <mo>+</mo>
        <msup>
          <mi>b</mi>
          <mn>2</mn>
        </msup>
      </mrow>
      <mo>=</mo>
      <msup>
        <mi>c</mi>
        <mn>2</mn>
      </msup>
    </mrow>
  </math>

  </body>
</html>