A=3^2/5x14+3^2/7x18+3^2/9x22+3^2/11x26+3^2/13x30 giup mik pls mik cấn gấp lắm

Tham khảo

` 4A=3^2.(\frac{2}{5.7}+\frac{2}{7.9}+....+\frac{2}{13.15})`

`⇒A=\frac{9}{4}.(\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{13}-\frac{1}{15})`

`⇒A=\frac{9}{4}.(\frac{1}{5}-\frac{1}{15})`

`⇒A=\frac{9}{4}.\frac{2}{15}`

`⇒A=\frac{3}{20}`

Giải thích

Áp dụng `\frac{2}{n(n+2)}=\frac{1}{n}-\frac{1}{n+2}(n \ne 0,-2)`

`\text{©CBT}`