求數學高手解答

2012-05-12 12:40 am
1. 不等式 (x^2-5x-24)/(x+a) <=0 的解集爲 {x|x<=-3 or 7<x<=8}, 求a值.
2.不等式 (mx)/(x-7)>2 的解集爲 {x|x<-1 or x>7}, 求實數a的值.

回答 (2)

2012-05-12 1:35 am
✔ 最佳答案
不等式 (x^2-5x-24)/(x+a)<=0 的解集爲 {x|x<=-3 or 7<x<=8}, 求a值.
Sol
(x^2-5x-24)/(x+a) <=0
(x^2-5x-24)(x+a)>=0,x+a<>0
(x-8)(x+3)(x=a)>=0,x<>-a
So
a=-7

不等式 (mx)/(x-7)>2 的解集爲{x|x<-1 or x>7}, 求實數m的值.
Sol
x<-1 or x>7
(x+1)(x-7)>0
mx/(x-7)>2
mx/(x-7)-2>0
(mx-2x+14)/(x-7)>0
(mx-2x+14)(x-7)>0
[(m-2)x+14](x-7)>0
[(m-2)x/14+1](x-7)>0
So
(m-2)/14=1
m=16


2012-05-12 12:56 am
1.
(x^2-5x-24)/(x+a) <=0

==>(x-8)(x+3)/(x+a)<=0

==>(x-8)(x+3)(x+a)<=0 ,x≠-a


解集爲 {x|x<=-3 or 7<x<=8}

==>(x-8)(x-7)(x+3)<=0 ,x≠7

比較知: a= -7


2012-05-11 17:09:51 補充:
2.請板主檢視是否有錯
應該為不等式 (mx)/(x-6)>2 的解集爲 {x|x<-1 or x>7}, 求實數a的值

mx(x-6)>2

==>mx^2-6mx-2>0

解集爲 {x|x<-1 or x>7}

==>(x+1)(x-7)>0

==>x^2-6x-7>0

==>(2/7)*(x^2-6x-7)>0

==>[(2/7)^x^2-(12x/7)-2]>0

比較係數知; m=2/7

2012-05-11 17:49:57 補充:
第二題看錯更正
mx/(x-7) -2>0

==>[(m-2)x+14]*(x-7)>0
==>(m-2)x^2-7(m-4)-98>0

解集爲 {x|x<-1 or x>7}
==>(x-7)(x+1)>0
==>x^2-6x-7>0

==>14x^2-84x-98>0

比較係數知:m-2=14 ==>m=16


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