A=1/3.7+1/7.11+1/11.15+...+1/91.95
=>4A=4/3.7+4/7.11+4/11.15+...+4/91.95
4A=1/3-1/7+1/7-1/11+1/11-1/15+...1/91-1/95
4A=1/3-1/95
4A=92/285
A=92/285:4
A=23/285
Bạn chỉ cần lấy 1/3 trừ 1/95 là đc r, ko cần : 4 đâu
^^!
\(4A=\frac{4}{3\cdot7}+\frac{4}{7\cdot11}+...+\frac{4}{91\cdot95}\)
\(4A=\frac{1}{3}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+...+\frac{1}{91}-\frac{1}{95}\)
\(4A=\frac{1}{3}-\frac{1}{95}\)
\(4A=\frac{92}{285}\)
\(A=\frac{23}{285}\)