Ta có:
Nếu `A = 15x - 23y ⋮13`
`=> 15x - 23y ⋮ 13`
`=> 15x - 23y - 13(x-2y) ⋮13` (vì `13(x-2y)⋮13`)
`=> 15x - 23y - 13x + 26y ⋮ 13`
`=> 2x + 3y ⋮ 13`
`=> B ⋮13`
Nếu `B = 2x + 3y ⋮ 13`
`=> 2x + 3y + 13(x-2y) ⋮13` (vì `13(x-2y) ⋮ 13`)
`=> 2x + 3y + 13x - 26y ⋮ 13`
`=> 15x - 23y ⋮ 13`
`=> A ⋮13`