`A = 4(3^2 + 1)(3^4 + 1) ... (3^64 + 1)`
`2A = 8(3^2 + 1)(3^4 + 1) ... (3^64 + 1)`
`2A = (3^2 - 1)(3^2 + 1)(3^4 + 1) ... (3^64 + 1)`
`2A = (3^4 - 1)(3^4 + 1)(3^8 + 1) ... (3^64 + 1)`
`2A = (3^8 - 1)(3^8 + 1)(3^16 + 1) ... (3^64 + 1)`
`2A = (3^16 - 1)(3^16 + 1)(3^32 + 1) ... (3^64 + 1)`
`2A = (3^32 - 1)(3^32 + 1)(3^64 + 1)`
`2A = (3^64 - 1)(3^64 + 1)`
`2A = 3^128 - 1`
`A = (3^128 - 1)/2`
`Vì: (3^128 - 1)/2 < 3^128 - 1`
`=> A < B`
`Vậy: A = 4(3^2 + 1)(3^4 + 1) ... (3^64 + 1) < B = 3^128 - 1`