- Ta có :
`A=3.6.9+6.9.12+9.12.15+12.15.18`
`A=(3.1)(3.2)(3.3)+(3.2)(3.3)(3.4)+(3.3)(3.4)(3.5)+(3.4)(3.5)(3.6)`
`A=(3.3.3)(1.2.3)+(3.3.3)(2.3.4)+(3.3.3)(3.4.5)+(3.3.3)(4.5.6)`
`A=27(1.2.3)+27(2.3.4)+27(3.4.5)+27(4.5.6)`
`A=27(1.2.3+2.3.4+3.4.5+4.5.6)`
`A=27B`
- Vậy `A>B` và `A=27B`