<?xml version="1.0" encoding="UTF-8"?>
<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
   <semantics xmlns="">
      <mrow>
         <mi>n</mi>
         <mo>=</mo>
         <msubsup>
            <mi>x</mi> 
            <mn>1</mn> 
            <mn>2</mn>
         </msubsup>
         <mo>+</mo>
         <msubsup>
            <mi>x</mi> 
            <mn>2</mn> 
            <mn>2</mn>
         </msubsup>
         <mo>+</mo>
         <mi>…</mi>
         <mo>+</mo>
         <msubsup>
            <mi>x</mi> 
            <mi>k</mi> 
            <mn>2</mn>
         </msubsup>
      </mrow>
      <annotation encoding="application/x-tex">n=x_{1}^{2}+x_{2}^{2}+\dots+x_{k}^{2}</annotation>
   </semantics>
</math>
