==> 2A = 2/3x5 + 2/5x7 +....+2/61x63
2A = 1/3-1/5+1/5-1/7+....+1/61-1/63
2A = 1/3-1/63
2A=20/63
==> A=10/63
\(A=\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+...+\frac{1}{61.63}\)
\(2A=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{61}-\frac{1}{63}\)
\(2A=1-\frac{1}{63}\)
\(2A=\frac{62}{63}\)
\(\Rightarrow A=\frac{31}{63}\)
Ta có: A= 1/3x5+1/5x7+1/7x9+...+1/61x63
Suy ra 2A= 2/3x5+2/5x7+2/7x9+....+2/61x63
2A= 1/3-1/5+1/5-1/7+1/7-1/9+...+1/61-1/63
2A= 1/3-1/63
2A= 20/63
Suy ra A= 20/63:2
A= 10/63
Vì Bastkoo làm tưởng hay là 1 chứ k phải 1/3 xin lỗi vì sự bất tiên này
1/3x5+1/5x7+1/7x9+...+1/37x39