Đáp án:
a)
`1990 . 1990 - 1992 . 1988`
`= 1990 . (1988 + 2) - (1990 + 2) . 1988`
`= (1990 . 1988 + 1990 . 2) - (1990 . 1988 + 2 . 1988)`
`= 1990 . 1988 + 1990 . 2 - 1990 . 1988 - 2 . 1988`
`= 1990 . 2 - 2 . 1988`
`= 2 . (1990 - 1988)`
`= 2 . 2`
`= 4`
Chúc học tốt!!!