Đáp án:
`A=2^256-1`
Giải thích các bước giải:
`A=3.(2^2+1)(2^4+1)(2^8+1)....(2^128+1)`
`A=(2^2-1)(2^2+1)(2^4+1)(2^8+1).....(2^128+1)`
`A=(2^4-1)(2^4+1)(2^8+1).......(2^128+1)`
`A=(2^8-1)(2^8+1).....(2^128+1)`
........
`A=(2^128-1)(2^128+1)`
`A=2^256-1`