Tham khảo
Sửa :`\frac{1}{4}+\frac{1}{12}+\frac{1}{36}+...+\frac{1}{972}`
Đặt `A=\frac{1}{4}+\frac{1}{12}+\frac{1}{36}+...+\frac{1}{972}`
`⇒\frac{1}{3}A=\frac{1}{12}+\frac{1}{36}+....+\frac{1}{2916}`
`⇒A-\frac{1}{3}A=\frac{1}{4}+\frac{1}{12}+\frac{1}{36}+...+\frac{1}{972}-(\frac{1}{12}+\frac{1}{36}+....+\frac{1}{2916})`
`⇒\frac{2}{3}A=\frac{1}{4}-\frac{1}{2916}`
`⇒A=\frac{187}{729}:\frac{2}{3}`
`⇒A=\frac{91}{243}`
`\text{©CBT}`
