Question

A = 7/8 + 3/40 + 2/35 + 9/112 + 1/272 hãy làm câu này theo dãy số quy luật

 

Answer 1

\(A=\dfrac{7}{1.8}+\dfrac{3}{5.8}+\dfrac{2}{5.7}+\dfrac{9}{7.16}+\dfrac{1}{16.17}=\)

\(=\dfrac{8-1}{1.8}+\dfrac{8-5}{5.8}+\dfrac{7-5}{5.7}+\dfrac{16-7}{7.16}+\dfrac{17-16}{16.17}=\)

\(=1-\dfrac{1}{8}+\dfrac{1}{5}-\dfrac{1}{8}+\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{16}+\dfrac{1}{16}-\dfrac{1}{17}=\)

\(=1-\dfrac{2}{8}+\dfrac{2}{5}-\dfrac{1}{17}\)