| 1978×1979+1979×1980 |
| 1+2+3+…+1978+1979+1978+1977+…+3+2+1 |
=
| 1979×(1978+1980) |
| (1+1978)×1978÷2×2+1979 |
=
| 1979×3958 |
| 1979×1978+1979 |
=
| 3958 |
| 1979 |
=2．
| 1978×1979+1979×1980 |
| 1+2+3+…+1978+1979+1978+1977+…+3+2+1 |
| 1978×1979+1979×1980 |
| 1+2+3+…+1978+1979+1978+1977+…+3+2+1 |
| 1979×(1978+1980) |
| (1+1978)×1978÷2×2+1979 |
| 1979×3958 |
| 1979×1978+1979 |
| 3958 |
| 1979 |