a) $\dfrac{-13}{7}.\dfrac{16}{35}.\dfrac{7}{-13}=(\dfrac{-13}{7}. \dfrac{7}{-13}).\dfrac{16}{35}=\dfrac{16}{35}$
b) $\dfrac{3}{4}.\dfrac{7}{25}+\dfrac{3}{4}.\dfrac{18}{25}=\dfrac{3}{4}.(\dfrac{7}{25}+\dfrac{18}{25})= \dfrac{3}{4}.1=\dfrac{3}{4}$
c) $\dfrac{7}{5}.\dfrac{8}{19}+\dfrac{7}{5}.\dfrac{12}{19}-\dfrac{7}{5}.\dfrac{1}{19}=\dfrac{7}{5}.( \dfrac{8}{19}+\dfrac{12}{19}-\dfrac{1}{19})=\dfrac{7}{5}.1=\dfrac{7}{5}$
d) $\dfrac{-2}{7}.\dfrac{5}{13}+\dfrac{-2}{7}.\dfrac{8}{13}+1\dfrac{1}{7}=\dfrac{-2}{7}.\dfrac{5}{13}+ \dfrac{-2}{7}.\dfrac{8}{13}+\dfrac{8}{7}=\dfrac{-2}{7}.(\dfrac{5}{13}+\dfrac{8}{13})+\dfrac{8}{7}=\dfrac{-2}{7}.1+\dfrac{8}{7}=\dfrac{6}{7}$
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