<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mtext>betac</mtext>
    <mfenced>
      <mrow>
        <mi>a</mi>
        <mo>,</mo>
        <mi>b</mi>
        <mo>,</mo>
        <mi>x</mi>
      </mrow>
    </mfenced>
    <mspace width="1em"/>
    <mo>=</mo>
    <mspace width="1em"/>
    <mn>1</mn>
    <mo>&#x2212;</mo>
    <msub>
      <mi>B</mi>
      <mi>x</mi>
    </msub>
    <mfenced>
      <mrow>
        <mi>a</mi>
        <mo>,</mo>
        <mi>b</mi>
      </mrow>
    </mfenced>
    <mspace width="1em"/>
    <mo>=</mo>
    <mspace width="1em"/>
    <msub>
      <mi>B</mi>
      <mrow>
        <mn>1</mn>
        <mo>&#x2212;</mo>
        <mi>x</mi>
      </mrow>
    </msub>
    <mfenced>
      <mrow>
        <mi>b</mi>
        <mo>,</mo>
        <mi>a</mi>
      </mrow>
    </mfenced>
  </mrow>
</math>