题目内容
先化简，再求值：当x=2，y=-1时，求
-
÷
的值．
| xy2 |
| x2-y2 |
| x4y |
| x4-y4 |
| x2 |
| x2+y2 |
原式=
-
•
=
-
=
=
=-
原式=-
=2．
| xy2 |
| x2-y2 |
| x4y |
| (x2-y2)•(x2+y2) |
| x2+y2 |
| x2 |
=
| xy2 |
| x2-y2 |
| x2y |
| x2-y2 |
=
| xy2-x2y |
| x2-y2 |
=
| xy(y-x) |
| (x-y)(x+y) |
=-
| xy |
| x+y |
原式=-
| 2×(-1) |
| 2+(-1) |
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