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Answer
\(\begin{array}{l}
\dfrac{9}{11} + \dfrac{5}{7} - \dfrac{20}{11} + \dfrac{8}{13} + \dfrac{2}{7}\\
= (\dfrac{9}{11} - \dfrac{20}{11}) + (\dfrac{5}{7} + \dfrac{2}{7}) + \dfrac{8}{13}\\
= \dfrac{-11}{11} + \dfrac{7}{7} + \dfrac{8}{13}\\
= -1 + 1 + \dfrac{8}{13}\\
= 0 + \dfrac{8}{13}\\
= \dfrac{8}{13}\\
\end{array}\)
`9/11 + 5/7 - 20/11 + 8/13 + 2/7`
`= ( 9/11 - 20/11 ) + ( 5/7 + 2/7 ) + 8/13`
`= (-11)/11 + 7/7 + 8/13`
`= -1 + 1 + 8/13`
`= 0 + 8/13`
`= 8/13`
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