58/7 - (31/9 + 30/7)
= 58/7 - 31/9 - 30/7
= (58/7 - 30/7) - 31/9
= 28/7- 31/9
= 4 - 31/9 = 36/9 - 31/9
= 5/9
478/19÷(-5/4)-668/19÷(-5/4)
=(478/19-668/19):(-5/4)
=-10 . (-4/5)
=8
a) \(=\dfrac{92}{9}-\dfrac{56}{9}+\dfrac{13}{5}\)
\(=4+\dfrac{15}{3}=\dfrac{12}{3}+\dfrac{15}{3}=9\)
b) \(=\dfrac{58}{7}-\dfrac{30}{7}-\dfrac{31}{9}\)
\(=4-\dfrac{31}{9}=\dfrac{36}{9}-\dfrac{31}{9}=\dfrac{5}{9}\)
c) \(=\dfrac{2}{3}+\dfrac{1}{3}.\dfrac{9}{4}-\dfrac{13}{12}\)
\(=\dfrac{1}{3}\left(2+\dfrac{9}{4}-\dfrac{13}{4}\right)\)
\(=\dfrac{1}{3}\left(2-1\right)=\dfrac{1}{3}.1=\dfrac{1}{3}\)
d) \(=\dfrac{41}{8}-\dfrac{35}{44}=\dfrac{381}{88}\)
e) \(\dfrac{478}{19}:\dfrac{5}{4}-\dfrac{668}{19}:\dfrac{5}{4}\)
\(=\dfrac{478}{19}.\dfrac{4}{5}-\dfrac{668}{19}.\dfrac{4}{5}\)
\(=\dfrac{4}{5}\left(\dfrac{478}{19}-\dfrac{668}{19}\right)=\dfrac{4}{5}.\left(-10\right)=-8\)
f) \(\dfrac{32}{21}.\dfrac{49}{45}:\dfrac{16}{22}.\dfrac{15}{33}\)
\(=\dfrac{32}{21}.\dfrac{49}{45}.\dfrac{11}{8}.\dfrac{5}{11}\)
\(=\dfrac{2^5.7^2.11.5}{7.3.3^2.5.2^3.11}=\dfrac{2^3.5.7.11\left(2^2.7\right)}{2^3.5.7.11\left(3^3\right)}\)
\(=\dfrac{28}{9}\)
g) Có: \(\dfrac{2020.2018+9}{2019.2020-2011}=\dfrac{2020\left(2019-1\right)+9}{2019.2020-2011}\)
\(=\dfrac{2020.2019-2020+9}{2020.2019-2011}=\dfrac{2020.2019-2011}{2020.2019-2011}\)\(=1\)
