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Không thì cô chê dốt

Bố sẽ rất buồn lòng.

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\(B=\dfrac{1}{6}+\dfrac{1}{12}+\dfrac{1}{20}+\dfrac{1}{30}+\dfrac{1}{42}+\dfrac{1}{56}+\dfrac{1}{72}\)

\(B=\dfrac{1}{2.3}+\dfrac{1}{3.4}+\dfrac{1}{4.5}+\dfrac{1}{5.6}+\dfrac{1}{6.7}+\dfrac{1}{7.8}+\dfrac{1}{8.9}\)

\(B=\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+...+\dfrac{1}{8}-\dfrac{1}{9}\)

\(B=\dfrac{1}{2}-\dfrac{1}{9}\)

\(B=\dfrac{9}{18}-\dfrac{2}{18}\)

\(B=\dfrac{7}{18}\)

\(B=\dfrac{-5}{9}+\dfrac{8}{15}+\dfrac{-2}{11}+\dfrac{4}{-9}+\dfrac{7}{15}\)

\(\text{​​}\text{​​}\text{​​}B=\dfrac{-5}{9}+\dfrac{8}{15}+\dfrac{-2}{11}+\dfrac{-4}{9}+\dfrac{7}{15}\)

\(B=(\dfrac{-5}{9}+\dfrac{-4}{9})+(\dfrac{8}{15}+\dfrac{7}{15})+\dfrac{-2}{11}\)

\(B=\dfrac{-9}{9}+\dfrac{15}{15}+\dfrac{-2}{11}\)

\(B=-1+1+\dfrac{-2}{11}\)

\(B=0+\dfrac{-2}{11}\)

\(B=\dfrac{-2}{11}\)

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