絕對值不等式

2012-08-26 1:57 am
I x+2 I< I 2x-4 I - 1
請教我這題應該如何解 需詳細

回答 (3)

2012-08-26 2:36 am
✔ 最佳答案
|x+2|<|2x-4|-1
Sol
x+2=0 =>x=-2
2x-4=0 =>x=2
(1) x<=-2
x+2<=0
|x+2|=-x-2
2x<=-4
2x-4<=-8
|2x-4|=-2x+4
|x+2|<|2x-4|-1
-x-2<-2x+4-1
x<5
So
x<=-2……………….
(2) -2<=x<=2
0<=x+2
|x+2|=x+2
2x<=4
2x-4<=0
|2x-4|=-2x+4
x+2<-2x+4-1
3x<1
x<1/3
So
-2<=x<1/3…………
(3) 2<=x
|x+2|=x+2
|2x-4|=2x-4
x+2<2x-4-1
7<x
So
x>7………………….
綜合(1)(2)(3)

x<=1/3 or x>7




2012-08-26 16:28:25 補充:
|x+2|+1<|2x-4|
x^2+4x+4+2|x+2|+1<4x^2-16x+16
3x^2-20x+11>2|x+2|
(1) x>=-2
3x^2-20x+11>2x+4
3x^2-22x+7>0
(x-1/3)(x-7)>0
x<1/3 or x>7
-2<=x<1/3 or x>7…………….
(2) x<=-2
3x^2-20x+11>-2x-4
3x^2-18x+15>0
(x-1)(x-5)>0
x<1 or x<5
x<=-2………………………..
綜合(1)(2)
x<1/3 or x>7
2012-08-26 11:17 am
加油! 加油!再加油!!
2012-08-26 2:52 am
設兩函數
y=I x+2 I
y=I 2x-4 I - 1
若x < -2
-2-x+1<4-2x==>x<5
{x < -2 且 x<5}={x< -2}
若-2<x<2
x+3<4-2x==>x<1/3
{-2<x<2 且 x<1/3}={-2<x<1/3}
若x>2
x+3<2x-4==>x>7
{x>2 且 x>7}={x>7}

所以取上面三個的聯集
{x< -2}或{-2<x<1/3}或{x>7}
{x< -2}或{-2<x<1/3}可以合併寫成x<1/3
即x<1/3或x>7
參考: 我


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