a)7/19.8/11+3/11.7/19+-12/19
=7/19.(8/11+3/11)+-12/19
=7/19.1+-12/19
=7/19+-12/19
=-5/19
a)
\(\dfrac{7}{19}\cdot\dfrac{8}{11}+\dfrac{3}{11}\cdot\dfrac{7}{19}+\dfrac{-12}{19}\\ =\dfrac{7}{19}\cdot\left(\dfrac{8}{11}+\dfrac{3}{11}\right)+\dfrac{-12}{19}=\dfrac{7}{19}\cdot1+\dfrac{-12}{19}=\dfrac{7}{19}+\dfrac{-12}{19}=\dfrac{5}{19}\)