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$\begin{array}{c}a+a+a+a\\ a+a+a+a\\ a+a+a+a\end{array}$

Source Code:

    <math> 
      <mtable> 
        <mtr> 
          <mtd> 
            <mi fontsize='14pt'>a</mi> 
            <mo color='black'>+</mo> 
            <mstyle fontsize='24pt' color='GREEN'> 
              <mi>a</mi> 
              <mo color='red'>+</mo> 
              <mi color='red'>a</mi> 
            </mstyle> 
            <mo>+</mo> 
            <mi>a</mi> 
          </mtd> 
        </mtr> 
        <mtr> 
          <mtd> 
            <mi mathsize='14pt'>a</mi> 
            <mo mathcolor='black'>+</mo> 
            <mstyle mathsize='24pt' mathcolor='GREEN'> 
              <mi>a</mi> 
              <mo mathcolor='red'>+</mo> 
              <mi mathcolor='red'>a</mi> 
            </mstyle> 
            <mo>+</mo> 
            <mi>a</mi> 
          </mtd> 
        </mtr> 
        <mtr> 
          <mtd> 
            <mi mathsize='14pt'>a</mi> 
            <mo mathcolor='black'>+</mo> 
            <mstyle mathsize='24pt' mathcolor='GREEN'> 
              <mi>a</mi> 
              <mo color='red'>+</mo> 
              <mi fontsize='14pt'>a</mi> 
            </mstyle> 
            <mo>+</mo> 
            <mi>a</mi> 
          </mtd> 
        </mtr> 
      </mtable> 
    </math>