Cảm ơn trước nhé 
Bài 1.
`@`\(A=\dfrac{5}{2.4}+\dfrac{5}{4.6}+\dfrac{5}{6.8}+...+\dfrac{5}{2020.2022}\)
\(A=\dfrac{5}{2}.\left(\dfrac{2}{2.4}+\dfrac{2}{4.6}+\dfrac{2}{6.8}+...+\dfrac{2}{2020.2022}\right)\)
\(A=\dfrac{5}{2}.\left(\dfrac{1}{2}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{8}+...+\dfrac{1}{2020}-\dfrac{1}{2022}\right)\)
\(A=\dfrac{5}{2}.\left(\dfrac{1}{2}-\dfrac{1}{2022}\right)\)
\(A=\dfrac{5}{2}.\dfrac{505}{1011}\)
\(A=\dfrac{2525}{2022}\)
`@`\(B=\dfrac{5}{5.10}+\dfrac{5}{10.15}+\dfrac{5}{15.20}+...+\dfrac{5}{2020.2025}\)
\(B=\dfrac{1}{5}-\dfrac{1}{10}+\dfrac{1}{10}-\dfrac{1}{15}+\dfrac{1}{15}-\dfrac{1}{20}+...+\dfrac{1}{2020}-\dfrac{1}{2025}\)
\(B=\dfrac{1}{5}-\dfrac{1}{2025}\)
\(B=\dfrac{404}{2025}\)
`@`\(C=\dfrac{7}{5.10}+\dfrac{7}{10.15}+\dfrac{7}{15.20}+\dfrac{7}{2025.2030}\)
\(C=\dfrac{7}{5}.\left(\dfrac{5}{5.10}+\dfrac{5}{10.15}+\dfrac{5}{15.20}+...+\dfrac{5}{2025.2030}\right)\)
\(C=\dfrac{7}{5}.\left(\dfrac{1}{5}-\dfrac{1}{10}+\dfrac{1}{10}-\dfrac{1}{15}+\dfrac{1}{15}-\dfrac{1}{20}+...+\dfrac{1}{2025}-\dfrac{1}{2030}\right)\)
\(C=\dfrac{7}{5}.\left(\dfrac{1}{5}-\dfrac{1}{2030}\right)\)
\(C=\dfrac{7}{5}.\dfrac{81}{406}\)
\(C=\dfrac{81}{290}\)
Bài 2.
`a)`\(-\dfrac{21}{13}x+\dfrac{1}{3}=-\dfrac{2}{3}\)
\(\Rightarrow-\dfrac{21}{13}x=-\dfrac{2}{3}-\dfrac{1}{3}\)
\(\Rightarrow-\dfrac{21}{13}x=-1\)
\(\Rightarrow x=\dfrac{13}{21}\)
