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15 tháng 12 2018
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3
6.1
\(\frac{3}{8}-\frac{4}{5}-\frac{-17}{40}=\frac{15}{40}-\frac{32}{40}+\frac{17}{40}=\frac{15+17-32}{40}=0\)
6.2
\(\frac{3}{4}-\frac{16}{32}+\frac{4}{-3}=\frac{9}{12}-\frac{6}{12}-\frac{16}{12}=\frac{9-6-16}{12}=\frac{-13}{12}\)
6.3
\(\frac{-4}{7}+\frac{2}{3}.\frac{-9}{14}=\frac{-4}{7}-\frac{3}{7}=-\frac{7}{7}=-1\)
6.4
\(8\frac{2}{7}-(3\frac{4}{9}+4\frac{2}{7})=8+\frac{2}{7}-(7+\frac{4}{9}+\frac{2}{7})=1-\frac{4}{9}=\frac{5}{9}\)
6.5
\((\frac{2}{3}-1\frac{1}{2}):\frac{4}{3}+\frac{1}{2}=\frac{-5}{6}.\frac{3}{4}+\frac{1}{2}=\frac{-5}{8}+\frac{1}{2}=\frac{-5}{8}+\frac{4}{8}=\frac{-1}{8}\)
6.6
\(\frac{-5}{13}+\frac{2}{5}+\frac{-8}{13}+\frac{3}{5}-\frac{3}{7}\\ =(\frac{-5}{13}+\frac{-8}{13})+(\frac{2}{5}+\frac{3}{5})-\frac{3}{7}\\ =-1+1-\frac{3}{7}=\frac{-3}{7}\)
Bài 8:
a/ $=(\frac{2}{3}-\frac{2}{3})+\frac{5}{7}=0+\frac{5}{7}=\frac{5}{7}$
b/ $=(\frac{5}{13}+\frac{8}{13})+\frac{-5}{7}+(\frac{-20}{41}+\frac{-21}{41})$
$=1-\frac{5}{7}-1=-\frac{5}{7}$
c/ $=\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+\frac{1}{9.11}+\frac{1}{11.13}$
$=\frac{1}{2}(\frac{5-3}{3.5}+\frac{7-5}{5.7}+\frac{9-7}{7.9}+\frac{11-9}{9.11}+\frac{13-11}{11.13})$
$=\frac{1}{2}(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}+\frac{1}{11}-\frac{1}{13})$
$=\frac{1}{2}(\frac{1}{3}-\frac{1}{13})$
$=\frac{5}{39}$