Tham khảo
Xét `A=\frac{1}{2^2}+\frac{1}{2^4}+...+\frac{1}{2^{100}}`
`⇒4A=1+\frac{1}{2^2}+...+\frac{1}{2^{98}}`
`⇒4A-A=1+\frac{1}{2^2}+...+\frac{1}{2^{98}}-(\frac{1}{2^2}+\frac{1}{2^4}+...+\frac{1}{2^{100}})`
`⇒3A=1-\frac{1}{2^{100}}`
`⇒3A=B`
`⇒\frac{A}{B}=\frac{1}{3}`
`\text{©CBT}`