\(\frac{5}{18.21}+\frac{5}{21.24}+\frac{5}{24.27}+...+\frac{5}{123.126}\)
\(=\frac{5}{3}\left(\frac{3}{18.21}+\frac{3}{21.24}+\frac{3}{24.27}+...+\frac{3}{123.126}\right)\)
\(=\frac{5}{3}\left(\frac{1}{18}-\frac{1}{21}+\frac{1}{21}-\frac{1}{24}+\frac{1}{24}-\frac{1}{27}+...+\frac{1}{123}-\frac{1}{126}\right)\)
\(=\frac{5}{3}\left(\frac{1}{18}-\frac{1}{126}\right)\)
\(=\frac{5}{3}.\frac{1}{21}\)
\(=\frac{5}{63}\)
Study well ! >_<
\(\frac{5}{18\times21}+\frac{5}{21\times24}+\frac{5}{24\times27}+...+\frac{5}{123\times126}\)
\(=5\times\left(\frac{1}{18\times21}+\frac{1}{21\times24}+...+\frac{1}{123\times126}\right)\)(đoạn này mk àm tắt nhé ^v^)
\(=5\times\frac{1}{3}\times\left(\frac{3}{18\times21}+...+\frac{3}{123\times126}\right)\)
\(=5\times\frac{1}{3}\times\left(\frac{1}{18}-\frac{1}{21}+...+\frac{1}{123}-\frac{1}{126}\right)\)
\(=\frac{5}{3}\times\left(\frac{1}{18}-\frac{1}{126}\right)\)
\(=\frac{5}{3}\times\frac{1}{21}=\frac{5}{63}\)
đúng hay sai mk ko biết đâu nhé!
