F.2 mathz
2006-12-10 9:04 pm
A stone is projected vertically upwards,so that its height above the ground after t sec. is (72t-16t^2).After what time is it 81 m above the ground?What does it strike the ground?
回答 (2)
2006-12-11 1:03 am
✔ 最佳答案
Time is 81 m above the ground72t - 16 t^2 = 81
16 t^2 - 72t + 81 = 0
(4t - 9)^2 = 0
4t - 9 = 0
4t = 9
t = 9 / 4
t = 2.25 (sec.)
Time to strike the ground (0 m above the ground)
72t - 16 t^2 = 0
16 t^2 - 72t = 0
t (16t - 72) = 0
t = 0 (rejected) or 16t - 72 = 0
Therefore, 16t - 72 = 0
16t = 72
t = 4.5 (sec.)
參考: Me
2006-12-10 9:09 pm
thats easy:
72t-16t^2=81
(9-4t)^2=0
t=2.25
72t-16t^2=81
(9-4t)^2=0
t=2.25
收錄日期: 2021-04-12 22:15:36
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20061210000051KK01809