| 1988+1989×1987 |
| 1988×1989−1 |
=
| 1988+1989×(1988−1) |
| 1988×1989−1 |
=
| 1988+1989×1988−1989 |
| 1988×1989−1 |
=
| 1988×1989−1 |
| 1988×1989−1 |
=1；
故答案为：1．
| 1988+1989×1987 |
| 1988×1989−1 |
| 1988+1989×1987 |
| 1988×1989−1 |
| 1988+1989×(1988−1) |
| 1988×1989−1 |
| 1988+1989×1988−1989 |
| 1988×1989−1 |
| 1988×1989−1 |
| 1988×1989−1 |