\(S=\frac{1}{2.5}+\frac{1}{5.8}+...+\frac{1}{17.20}\)
\(\Rightarrow3S=\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+...+\frac{1}{17}-\frac{1}{20}\)
\(\Rightarrow3S=\frac{1}{2}-\frac{1}{20}\)
\(\Rightarrow3S=\frac{9}{20}\)
\(\Rightarrow S=\frac{3}{20}\)
\(S=\frac{1}{2\cdot5}+\frac{1}{5\cdot8}+...+\frac{1}{17\cdot20}\)
\(S=\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+...+\frac{1}{17}-\frac{1}{20}\)
\(S=\frac{1}{2}-\frac{1}{20}\)
\(S=\frac{9}{20}\)
\(S=\frac{1}{2.5}+\frac{1}{5.8}+...+\frac{1}{17.20}\)
\(3S=\frac{3}{2.5}+\frac{3}{5.8}+...+\frac{3}{17.20}\)
\(3S=\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+...+\frac{1}{17}-\frac{1}{20}\)
\(3S=\frac{1}{2}-\frac{1}{20}\)
\(3S=\frac{10}{20}-\frac{1}{20}\)
\(3S=\frac{9}{20}\)
\(S=\frac{9}{20}:3\)
\(S=\frac{9}{20}.\frac{1}{3}\)
\(S=\frac{3}{20}\)
\(S=\frac{1}{2\times5}+\frac{1}{5\times8}+...+\frac{1}{17\times20}\)
\(S=\frac{1}{3}\times\left(\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+...+\frac{1}{17}-\frac{1}{20}\right)\)
\(S=\frac{1}{3}\times\left(\frac{1}{2}-\frac{1}{20}\right)\)
\(S=\frac{1}{3}\times\frac{9}{20}\)
\(S=\frac{3}{20}\).
~ Chúc bạn hok tốt ~
\(S=\frac{1}{2.5}+\frac{1}{5.8}+..........+\frac{1}{17.20}\)
\(\Rightarrow3S=\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-...........-\frac{1}{20}\)
\(\Rightarrow3S=\frac{1}{2}-\frac{1}{20}\)
\(\Rightarrow3S=\frac{10}{20}-\frac{1}{20}\)
\(\Rightarrow3S=\frac{9}{20}\)
\(\Rightarrow S=\frac{3}{20}\)
Vậy S = \(\frac{3}{20}\)
P.s: Phải có kết luận nha em
\(S=\frac{1}{2\cdot5}+\frac{1}{5\cdot8}+...+\frac{1}{17\cdot20}\)
\(3S=\frac{3}{2\cdot5}+\frac{3}{5\cdot8}+...+\frac{3}{17\cdot20}\)
\(3S=\frac{1}{2\cdot5}+\frac{1}{5\cdot8}+...+\frac{1}{17\cdot20}\)
\(3S=\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+...+\frac{1}{17}-\frac{1}{20}\)
\(3S=\frac{1}{2}-\frac{1}{20}\)
\(3S=\frac{10}{20}-\frac{1}{20}\)
\(3S=\frac{9}{20}\)
\(S=\frac{9}{20}:3\)
\(S=\frac{3}{20}\)
