2/5 x 7 + 2/7 x9 + 2 /9 x 11 + ... + 2 /13 x 15
= 1 /5 - 1 /7 + 1/7 - 1/9 + 1/9 - 1/11 + ... + 1 /13 - 1/15
= 1/5 - 1/15
= 2/15
Vậy đáp án là B nhé
Đặt \(A=\frac{2}{5\cdot7}+\frac{2}{7\cdot9}+...+\frac{2}{13\cdot15}\)
Ta có : \(2A=\frac{2}{5}-\frac{2}{7}+\frac{2}{7}-\frac{2}{9}+...+\frac{2}{13}-\frac{2}{15}\)
\(2A=\frac{2}{5}-\frac{2}{15}\)
\(2A=\frac{4}{15}\)
\(A=\frac{4}{15}:2\)
\(A=\frac{2}{15}\)