Câu 2: Tình:

\(B=\dfrac{10}{56}+\dfrac{10}{140}+\dfrac{10}{260}+...+\dfrac{10}{1400}\)

\(B=\dfrac{5}{28}+\dfrac{5}{70}+\dfrac{5}{130}+...+\dfrac{5}{700}\)

\(B=\dfrac{5}{4.7}+\dfrac{5}{7.10}+\dfrac{5}{10.13}+...+\dfrac{5}{25.28}\)

\(B=3.5\left(\dfrac{1}{7}-\dfrac{1}{10}+\dfrac{1}{10}-\dfrac{1}{13}+...+\dfrac{1}{25}-\dfrac{1}{28}\right)\)

\(B=15\left(\dfrac{1}{4}-\dfrac{1}{28}\right)\)

\(B=15.\dfrac{3}{14}\)

\(B=\dfrac{45}{14}\)

Chia dạng đúng của từ:

1. Is your mother a ....teacher?.... (teach)

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