Inequalities

2013-02-11 4:27 am
Q1) If kx^2+2x+k<0 for all real values of x,
where k≠0, find the range of the values of k.

Answer: k<-1

Q2) If kx^2-x+k>0 for all real values of x,
where k≠0, find the range of the values of k.

Answer: k>1/2

How to find out the answer?

Please explain thoroughly!!

Thank you!
更新1:

點解圖形與x軸不相交既??

回答 (2)

2013-02-11 5:09 am
✔ 最佳答案
Q1) If kx^2+2x+k<0 for all real valuesof x,
where k≠0,find therange of the values of k.
Sol
kx^2+2x+k<0 for all real values of x
k<0…………………..(1)
圖形開口向下,恆小於0
圖形與x軸不相交
判別式<0
D=2^2-4*k*k<0
4-4k^2<0
k^2-1>0
(k-1)(k+1)>0
k<-1 or k>1………….(2)
綜合(1),(2)
k<-1

Q2) If kx^2-x+k>0 forall real values of x,
where k≠0, find the range of the values of k.
Sol
kx^2-x+k>0 forall real values of x
k>0……………………….
圖形開口向上,恆大於0
圖形與x軸不相交
判別式<0
D=(-1)^2-4*k*k<0
1-4k^2<0
4k^2-1>0
(2k-1)(2k+1)>0
(k-1/2)(k+1/2)>0
k<-1/2 ork>1/2…………..
綜合(1),(2)
k>1/2


2013-02-11 6:32 am
設y=kx^2+2x+k
y<0 for all real values of x, 即相對於任何的x值,y值都少於0
由此可見y=kx^2+2x+k 的圖像與x軸不相交 (開口向下)


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