小专题(十六) 分式的运算
题组1 分式的混合运算
1.计算:
(1)a a +1+a -1a 2-1
; 解:原式=a a +1+a -1(a +1)(a -1)
=a a +1+1a +1
=a +1a +1
=1.
(2)2x x 2-1÷x x +1
; 解:原式=2x x 2-1·x +1x
=2x (x +1)(x -1)·x +1x
=2x -1
. (3)(x x -2-x x +2)÷4x x -2
; 解:原式=4x (x +2)(x -2)·x -24x
=1x +2
.
(4) 2a a +1-2a -4a 2-1÷a -2a 2-2a +1
; 解:原式=2a a +1-2(a -2)(a -1)(a +1)·(a -1)2
a -2
=2a a +1-2(a -1)a +1
=2a +1
.
(5)(1+2x -1)÷x +1x 2-2x +1
; 解:原式=x -1+2x -1·(x -1)2
x +1
=x +1x -1·(x -1)2
x +1
=x -1.
(6) (a +2-5a -2)·2a -43-a
; 解:原式=a 2-4-5a -2·2a -43-a
=(a +3)(a -3)a -2·2(a -2)3-a
=-2(a +3)
=-2a -6.
(7)2-x x -1÷(x +1-3x -1
);
解:原式=2-x x -1÷x 2-4x -1
=-x -2x -1·x -1(x +2)(x -2)
=-1x +2
.
(8)(12-x +1)÷x -3x 2-4·x x 2+4x +4
. 解:原式=3-x 2-x ·(x +2)(x -2)x -3·x (x +2)2
=x x +2
.
题组2 分式的化简求值
2.先化简,再求值:(1+1x -1)÷x 2
,其中x =2 016. 解:原式=x x -1÷x 2=x x -1·2x =2x -1
. 当x =2 016时,原式=22 016-1=22 015
.
3.先化简,再求值:(6x -1+4x 2-1)÷3x +2x -1
,其中x =2. 解:原式=[6(x +1)(x +1)(x -1)+4(x +1)(x -1)]·x -13x +2
=6x +10(x +1)(x -1)·x -13x +2
=6x +10(x +1)(3x +2)
. 当x =2时,原式=12+1024=1112
.
4.先化简,再求值:(a +1a +2)÷(a -2+3a +2
),其中a 满足a -2=0. 解:原式=a (a +2)+1a +2÷a 2-4+3a +2
=(a +1)2a +2·a +2(a +1)(a -1)
=a +1a -1
. 当a -2=0,即a =2时,原式=3.
5.化简并求值:(1x -y +1x +y )÷2x -y x 2-y 2,其中x .y 满足|x -2|+(2x -y -3)2=0.
解:∵|x -2|+(2x -y -3)2=0,
∴?????x -2=0,2x -y -3=0, 解得?????x =2,y =1.
∴原式=x +y +x -y (x -y )(x +y )·(x +y )(x -y )2x -y =2x 2x -y
. 当x =2,y =1时,原式=43
.
6.先化简,再求值:(1-3x +2)÷x -1x 2+2x -x x +1
,其中x 满足x 2-x -1=0.
解:原式=x +2-3x +2·x (x +2)x -1-x x +1
=x -x x +1
=x 2
x +1
. ∵x 2-x -1=0,∴x 2=x +1.∴原式=1.
7.化简:2x x +1-2x +4x 2-1÷x +2x 2-2x +1
,然后在不等式x ≤2的非负整数解中选择一个适当的数代入求值.
解:原式=2x x +1-2(x +2)(x +1)(x -1)·(x -1)2
x +2
=2x x +1-2x -2x +1
=2x -2x +2x +1
=2x +1
. ∵不等式x ≤2的非负整数解是0,1,2.
答案不唯一,如:
当x =0时,原式=20+1
=2; 当x =1时,原式=21+1
=1; 当x =2时,原式=22+1=23
.
8.化简求值:a 2-6ab +9b 2a 2-2ab ÷(5b 2a -2b -a -2b )-1a ,其中a ,b 满足?
????a +b =4,a -b =2. 解:原式=(a -3b )2a 2-2ab ÷9b 2-a 2a -2b -1a
=-(a -3b )2a (a -2b )·a -2b (a -3b )(a +3b )-1a
=a -3b -a (a +3b )-1a
=-2a +3b
. ∵a ,b 满足?????a +b =4,a -b =2.∴?
????a =3,b =1. ∴原式=-23+3=-13
.
9.先化简,再求值:(x x 2+x -1)÷x 2-1x 2+2x +1
,其中x 的值从不等式组?????-x ≤1,2x -1<4
的整数解中选取. 解:原式=x -x 2-x x (x +1)÷(x +1)(x -1)(x +1)2
=-x 2x (x +1)·(x +1)2
(x +1)(x -1)
=-x x -1
. 解?????-x ≤1,2x -1<4,
得-1≤x <52, ∴不等式组的整数解为x =-1,0,1,2,要使分式有意义,x 只能取2,
∴原式=-22-1
=-2.
10.先化简:x 2+x x 2-2x +1÷(2x -1-1x
),再从-2<x <3的范围内选取一个你喜欢的x 值代入求值.
解:原式=x (x +1)(x -1)2÷2x -(x -1)x (x -1)
=x (x +1)(x -1)2·x (x -1)x +1
=x 2
x -1
. 取x =2,当x =2时,原式=x 2x -1=22
2-1=4.(答案不唯一.注:x ≠±1,0)