题目内容
先化简，再求值：(
-
)-
，其中a=
．
| a2-1 |
| a-1 |
| ||
| a2+a |
| 1 |
| a |
| 2 |
| tan45°-tan60° |
分析：首先化简a与所求的代数式，代入求值即可．
解答：解：a=
=-
-1
∴a+3=-
＜0
原式=
-
-
=a+1-
-
=a+1+
-
=a+1
=-
．
| 2 | ||
1-
|
| 3 |
∴a+3=-
| 3 |
原式=
| (a+1)(a-1) |
| a-1 |
| ||
| a(a+1) |
| 1 |
| a |
=a+1-
| |a+1| |
| a(a+1) |
| 1 |
| a |
=a+1+
| 1 |
| a |
| 1 |
| a |
=a+1
=-
| 3 |
点评：本题考查了代数式的化简求值，关键是对代数式进行化简．
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