quadratic equation problems

1.
h = 1 + 4t - 5t²

When h = 0 :
1 + 4t - 5t² = 0
(1 + 5t)(1 - t) = 0
t = -1/5 (rejected) or t = 1

The ball will reach the ground in 1s.


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2.
M = 2n(n - 1)

When M = 760 :
2n(n - 1) = 760
n(n - 1) = 380
n² - n - 380 = 0
(n + 19)(n - 20) = 0
n = -19 (rejected) or n = 20

There are 20 teams in the footballleague.


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3.
Let u be the units digit.
Then, the tens digit = u - 3

u(u - 3) = 18
u² - 3u = 18
u² - 3u - 18 = 0
(u - 6)(u + 3) = 0
u = 6 or u = -3 (rejected)
u - 3 = 6

The number is 36.

2014-08-31 09:24:16 補充：
3.
The last second line is : u - 3 = 3

你好!
以下是詳細圖解 :

Q1 :
http://i.imgur.com/dEAdYwg.png

Q2 :
http://i.imgur.com/0ObrydO.png

Q3 :
http://i.imgur.com/XUSPl8t.png