maths( f.5 )

2010-10-12 5:48 am

A bamboo stick of length 76cm is cut into two pieces The pieces are tied at
right angles to each other to form the skeleton of a kite .One of the sticks is x cm long and the two sides of the kite are symmetrical.

http://img99.imageshack.us/i/imgam.jpg/

a)Express the area of the kite in terms of x.

b)If the area of the kite is not less than 720cm^2,find the range of values of x.

更新1:

TO LUCY:都唔啱嘅

更新2:

To ee*** :都唔係好啱

回答 (2)

[匿名]

2010-10-12 6:16 am

✔ 最佳答案

a) Let the vertical stick be x and the horizontal one will be 76-x.

Area of the kite: 2x〔( 76-x )/ 2 〕
=76x-x^2
------------------------------------------------------------------------------------------------------------
b) 76x - x^2 ≥ 720
x^2 - 76x + 720 ≤ 0
11.1 ≤ x ≤ 64.9 (corr. to 3 sig. fig.)

2010-10-11 22:57:48 補充：
Sorry for texting wrong answer
I think this might be right:

2010-10-11 22:57:53 補充：
Let the vertical stick be x and the horizontal one will be 76-x.

Area of the kite: 2(x〔( 76-x )/ 2 〕)/2
=38x-1/2x^2
------------------------------------------------------------------------------------------------------------
b) 38x-1/2x^2 ≥ 720
x^2-76x+1440 ≤ 0
36 ≤ x ≤ 40

👍 0 👎 0

用戶10012

2010-10-12 6:23 am

Let the horizontal stick = x cm. ( In fact it does not matter which stick is assumed to be x cm.)
so the vertical stick = (76 - x) cm. long.
Area of kite = 2 x Area of triangle with height (x/2) and base (76 - x)
= 2[(x/2)(76 - x)]/2 = x(76 - x)/2
Now x(76 - x)/2 > 720
76x - x^2 > 1440
x^2 - 76x + 1440 < 0
(x - 40)(x - 36) < 0
so 36 < x < 40.

👍 0 👎 0

收錄日期: 2021-04-25 22:49:55
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20101011000051KK01446

檢視 Wayback Machine 備份