A = \(\frac{1}{10}=0,1\)
Tích cho tớ để đủ 250 điểm đi
=> A = 1/6*7 + 1/7*8 + 1/8*9 + 1/9*10 + ... + 1/13*14 + 1/14*15 = 1/6 - 1/7 + 1/7 - 1/8 + 1/8 - 1/9 + 1/9 - 1/10 +...+ 1/13 - 1/14 + 1/14 - 1/15 = 1/6 - 1/15 =1/10 Nhớ ^^
\(A=\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+\frac{1}{90}+\frac{1}{110}+\frac{1}{132}+\frac{1}{156}+\frac{1}{182}+\frac{1}{210}\)
\(\Rightarrow A=\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+\frac{1}{9.10}+\frac{1}{10.11}+\frac{1}{11.12}+\frac{1}{12.13}+\frac{1}{13.14}+\frac{1}{14.15}\)
\(\Rightarrow A=\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-....-\frac{1}{15}\)
\(\Rightarrow A=\frac{1}{6}.\left(-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-....-\frac{1}{14}+\frac{1}{14}\right)-\frac{1}{15}\)
\(\Rightarrow A=\frac{1}{6}\left(0+0....+0\right)-\frac{1}{15}\)
\(\Rightarrow A=\frac{1}{6}-\frac{1}{15}\)
\(\Rightarrow A=\frac{1}{10}\)
Vâng, em nó đã nhìn thấy trăng sao bay trên đầu khi đánh xong bài này :-))
