數學inequalities 高手請進

Please show the questions, then many people can help you.

2013-11-10 16:25:22 補充：
OK, Yvonna, I am here now.
Let me try it...

2013-11-10 17:01:45 補充：
Question 17
We need:
{ a + b > c
{ a + c > b
{ b + c > a

Now,
{ 6(x+1) + (x+4) > 2x+5
{ 6(x+1) + (2x+5) > x + 4
{ (x+4) + (2x+5) > 6(x+1)

{ 6x+6 + x+4 > 2x+5
{ 6x+6 + 2x+5 > x + 4
{ x+4 + 2x+5 > 6x+6

{ 7x+10 > 2x+5
{ 8x+11 > x + 4
{ 3x+9 > 6x+6

{ 5x > -5
{ 7x > -7
{ 3 > 3x

{ x > -1
{ x > -1
{ 1 > x

-1 < x < 1


Question 18
Let n be the number of sides.
Sum of all interior angles = (n-2) * 180°
Sum of all exterior angles = 360°

(n-2) * 180° - 360° ≥ 300°

(n-2) * 180° ≥ 660°

n - 2 ≥ 3.6666667

n ≥ 5.6666667

n ≥ 6

The polygon has at least 6 sides.


Question 19
Out of the 1000 cm³ of liquid taken from bottle A,
300 cm³ is alcohol and 700 cm³ is water.

Let x cm³ be the amount of liquid should be taken from bottle B.
In there 0.9x cm³ is alcohol and 0.1x cm³ is water.

In the mixed liquid, the total amount is (1000+x) cm³.
The amount of alcohol is (300+0.9x) cm³.

It is required that
(300+0.9x) / (1000+x) ≥ 50%
(300+0.9x) / (1000+x) ≥ 1/2
2(300+0.9x) ≥ 1000+x
600 + 1.8x ≥ 1000 + x
0.8x ≥ 400
x ≥ 500

That is, at least 500 cm³ of liquid B is to be taken out.


Question 20
Let x min be the amount of airtime William used last month.
It is given that x < 350.

$88 + $0.6(x - 200) > $138
88 + 0.6x - 120 > 138
88 + 0.6x > 258
0.6x > 170
x > 283.333

The least amount of airtime he used last month is 284 min.
(This is because time less than 1 min is counted as 1 min.)

2013-11-10 21:50:23 補充：
My pleasure.

for more clear image. Plz surf this web

http://imgur.com/losIlDz

Hope you can help me to solve these problems

2013-11-10 15:21:56 補充：
Plz tell him to help me. I really need help.

抄D題目出嚟睇睇啦！愛莫能助呀！

2013-11-10 00:04:38 補充：
用 x^2 代表 x²

1/(1+x^2) 代表 (1+x²) 分之 1

就好多人睇得明架啦！

2013-11-10 11:25:14 補充：
file:///C:/Users/Yvonna/Dropbox/%E7%9B%B8%E6%A9%9F%E4%B8%8A%E5%82%B3/2013-11-09%2023.06.36.jpg

小朋友 : 這是在你的硬碟 C 上的檔案， [ file:///C:/Users/Yvonna/Dropbox/ ]，點睇呀！

2013-11-10 12:41:31 補充：
Sorry, I am busy today. But I think masterijk will help you later.

2013-11-10 21:00:09 補充：
Thank you Masterijk .