\(\dfrac{8}{9}.-\dfrac{7}{13}-\dfrac{8}{9}.\dfrac{6}{13}+2\dfrac{8}{9}\\ =\dfrac{8}{9}.\left(-\dfrac{7}{13}-\dfrac{6}{13}\right)+\dfrac{26}{9}\\ =\dfrac{8}{9}.-1+\dfrac{26}{9}\\ =-\dfrac{8}{9}+\dfrac{26}{9}\\ =2.\)

\(x\times40+x\times\dfrac{3}{5}-x:\dfrac{1}{5}=360\\ x\times40+x\times\dfrac{3}{5}-x\times5=360\\ x\times\left(40+\dfrac{3}{5}-5\right)=360\\ x\times\dfrac{178}{5}=360\\ x=\dfrac{900}{89}.\)

Đáp án: He becomes a good english teacher.

\(a,\dfrac{5}{3}.x-\dfrac{2}{5}=\dfrac{1}{4}\\ \Rightarrow\dfrac{5}{3}.x=\dfrac{13}{20}\\\Rightarrow x=\dfrac{39}{100}\\ b,\dfrac{4}{7}.x-\dfrac{8}{5}=\dfrac{1}{2}\\ \Rightarrow\dfrac{4}{7}.x=\dfrac{21}{10}\\ \Rightarrow x=\dfrac{147}{40}. \)

\(346\times458+346+346\times541\\ =346\times\left(458+1+541\right)\\ =346\times1000\\ =346000.\)

\(\dfrac{2}{5}\times\dfrac{9}{5}+\dfrac{1}{3}:\dfrac{2}{6}\\ =\dfrac{18}{25}+\dfrac{1}{3}\times\dfrac{6}{2}\\ =\dfrac{18}{25}+1\\ =\dfrac{18}{25}+\dfrac{25}{25}\\ =\dfrac{43}{25}\)

\(-\dfrac{3}{29}-\dfrac{16}{28}\\ =-\dfrac{137}{203}.\)

Đáp án: A. Kẻ dọc, hoa nhỏ.

Sửa 2 dòng cuối:

\(2A=\dfrac{98}{101}\\ A=\dfrac{49}{101}\)

\(A=\dfrac{3}{3\times5}+\dfrac{3}{5\times7}+\dfrac{3}{7\times9}+...+\dfrac{3}{99\times101}\\2A=\dfrac{3\times2}{3\times5}+\dfrac{3\times2}{5\times7}+...+\dfrac{3\times2}{99\times101}\\ 2A=3\times\left(\dfrac{2}{3\times5}+\dfrac{2}{5\times7}+...+\dfrac{2}{99\times101}\right)\\ 2A=3\times\left(\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+...+\dfrac{1}{99}-\dfrac{1}{101}\right)\\ 2A=3\times \left(\dfrac{1}{3}-\dfrac{1}{101}\right)\\ 2A=3\times\dfrac{98}{303}\\ 2A=9898\\ A=4949.\)