Câu 13:
a) \(\dfrac{1}{4}+\dfrac{5}{12}-\dfrac{7}{6}\) b) \(\dfrac{-3}{7}.\dfrac{2}{11}+\dfrac{-3}{7}.\dfrac{9}{11}+1\dfrac{3}{7}\)
\(=\dfrac{3}{12}+\dfrac{5}{12}-\dfrac{7}{6}\) \(=\dfrac{-3}{7}.\left(\dfrac{2}{11}+\dfrac{9}{11}\right)+\dfrac{10}{7}\)
\(=\dfrac{8}{12}-\dfrac{7}{6}\) \(=\dfrac{-3}{7}.1+\dfrac{10}{7}\)
\(=\dfrac{8}{12}-\dfrac{14}{12}\) \(=\dfrac{-3}{7}+\dfrac{10}{7}\)
\(=\dfrac{-6}{12}=\dfrac{-1}{2}\) \(=1\)
c) 5.(-3)2+4.(-7)-(-16)
= 5.9+4.(-7)-(-16)
= 45+4.(-7)-(-16)
= 45+(-28)-(-16)
= 17-(-16)
= 33
Câu 14:
\(x+\dfrac{1}{3}=\dfrac{1}{12}\)
\(x=\dfrac{1}{12}-\dfrac{1}{3}\)
\(x=\dfrac{1}{12}-\dfrac{4}{12}\)
\(x=\dfrac{-3}{12}=\dfrac{-1}{4}\)
Vậy \(x=\dfrac{-1}{4}\)
b) \(\dfrac{1}{5}.x-\dfrac{1}{20}=-\dfrac{1}{4}\)
\(\dfrac{1}{5}.x=-\dfrac{1}{4}+\dfrac{1}{20}\)
\(\dfrac{1}{5}.x=-\dfrac{5}{20}+\dfrac{1}{20}\)
\(\dfrac{1}{5}.x=-\dfrac{4}{20}=-\dfrac{1}{5}\)
\(x=-\dfrac{1}{5}:\dfrac{1}{5}\)
\(x=-\dfrac{1}{5}.\dfrac{5}{1}\)
\(x=-\dfrac{5}{5}=-1\)
Vậy \(x=-1\)
