\(\frac{1}{72}+\frac{1}{56}+\frac{1}{42}+\frac{1}{30}+\frac{1}{20}+\frac{1}{12}\)
\(=\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}\)
\(=\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}\)
\(=\frac{4-3}{3.4}+\frac{5-4}{4.5}+\frac{6-5}{5.6}+\frac{7-6}{6.7}+\frac{8-7}{7.8}+\frac{9-8}{8.9}\)
\(=\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}\)
\(=\frac{1}{3}-\frac{1}{9}\)
\(=\frac{3}{9}-\frac{1}{9}\)
\(=\frac{2}{9}\)
1/72 = 1/9.1/8; 1/56 = 1/8.1/7; 1/42 = 1/7.1/6; 1/30 = 1/6.1/5; 1/20 = 1/5.1/4; 1/12 = 1/4.1/3
1/9 - 1/8 + 1/8 - 1/7 + 1/7 - 1/6 + 1/6 - 1/5 + 1/5 - 1/4 + 1/4 - 1/3 = 1/9 - 1/3 = -2/9
Tớ nghĩ là vậy, tớ đã tìm thấy 1 bài tương tự như vậy trên mạng rồi.
Tớ dư dấu trừ rồi. Bằng 2/9 thôi nhá, xin lỗi
