$\frac{2}{1x3} + \frac{2}{2x4} + ... + \frac{2}{19x21} = 2a$
$(\frac{1}{1} -\frac{1}{3} ) + (\frac{1}{2} -\frac{1}{4} ) + (\frac{1}{3} -\frac{1}{5} ) +...+ (\frac{1}{19} -\frac{1}{21} ) = 2a$
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