| x−y |
| x |
| 2xy−y2 |
| x |
=
| x−y |
| x |
| x2 |
| x |
| 2xy−y2 |
| x |
=
| x−y |
| x |
| x2−2xy+y2 |
| x |
=
| x−y |
| x |
| (x−y)2 |
| x |
=
| x−y |
| x |
| x |
| (x−y)2 |
=
| 1 |
| x−y |
当x=2009，y=2010时，原式=
| 1 |
| x−y |
| 1 |
| 2009−2010 |
| x−y |
| x |
| 2xy−y2 |
| x |
| x−y |
| x |
| 2xy−y2 |
| x |
| x−y |
| x |
| x2 |
| x |
| 2xy−y2 |
| x |
| x−y |
| x |
| x2−2xy+y2 |
| x |
| x−y |
| x |
| (x−y)2 |
| x |
| x−y |
| x |
| x |
| (x−y)2 |
| 1 |
| x−y |
| 1 |
| x−y |
| 1 |
| 2009−2010 |