题目内容
当m=-
时，代数式
-
÷
-
的值是（ ）
| 1 |
| 6 |
| 21-5m |
| m2-9 |
| m |
| m2-9 |
| m |
| m+3 |
| m-3 |
| m+3 |
| A．-1 | B．-
| C．
| D．1 |
| 21-5m |
| m2-9 |
| m |
| m2-9 |
| m |
| m+3 |
| m-3 |
| m+3 |
=
| 21-5m |
| m2-9 |
| m |
| (m+3)(m-3) |
| m+3 |
| m |
| m-3 |
| m+3 |
=
| 21-5m |
| m2-9 |
| 1 |
| m-3 |
| m-3 |
| m+3 |
=
| 21-5m |
| (m+3)(m-3) |
| m-3 |
| (m+3)(m-3) |
| (m-3)(m-3) |
| (m+3)(m-3) |
=
| 21-5m-m-3-m2+6m-9 |
| (m+3)(m-3) |
=
| 9-m2 |
| (m+3)(m-3) |
=
| 9-m2 |
| m2-9 |
=-1．
故选A．
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