Question

giúp tôi bài này với:

A=1/6+2/15+3/40+1/72+2/99+3/154

Thanks nhiều 

Answer 1

Tự chép đề

\(A=\frac{1}{2\cdot3}+\frac{2}{3\cdot5}+\frac{3}{5\cdot8}+\frac{1}{8\cdot9}+\frac{2}{9\cdot11}+\frac{3}{11\cdot14}\)

\(A=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}+\frac{1}{11}-\frac{1}{14}\)

\(A=\frac{1}{2}-\frac{1}{14}\)

\(A=\frac{7}{14}-\frac{1}{14}=\frac{6}{14}=\frac{3}{7}\)

Vậy \(A=\frac{3}{7}\)