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<math xmlns="http://www.w3.org/1998/Math/MathML">
<mi>c</mi> <mo>=</mo>
 <mfrac>
<mrow>
<msub>
<mo> <mi>b</mi> </mo> <mo> <mn>1</mn> <mo/>
</msub> - <msub>
<mo> <mi>b</mi> </mo> <mo> <mn>2</mn> <mo/>
</msub>
</mrow>
<mrow>
<mn>2</mn>
</mrow>
</mfrac> <mo>=</mo>
 <mi>a</mi> 
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</math>
</p></body></html>