<?xml version="1.0"?>
<!DOCTYPE mrow SYSTEM "mathml.dtd" [
 <!ENTITY PlusMinus      "plus minus">
 <!ENTITY InvisibleTimes "*">
]> 
<!-- The formula for the solutions of a quadratic equation
      x = (-b +/- Sqrt(b^2 - 4ac))/2a
 -->
<mrow>
        <mi>x</mi>
        <mo>=</mo>
        <mfrac>
          <mrow>
            <mrow>
              <mo>-</mo>
              <mi>b</mi>
            </mrow>
            <mo>&PlusMinus;</mo>
            <msqrt>
              <mrow>
                <msup>
                  <mi>b</mi>
                  <mn>2</mn>
                </msup>
                <mo>-</mo>
                <mrow>
                  <mn>4</mn>
                  <mo>&InvisibleTimes;</mo>
                  <mi>a</mi>
                  <mo>&InvisibleTimes;</mo>
                  <mi>c</mi>
                </mrow>
              </mrow>
            </msqrt>
          </mrow>
          <mrow>
            <mn>2</mn>
            <mo>&InvisibleTimes;</mo>
            <mi>a</mi>
          </mrow>
        </mfrac>
      </mrow>