急......QUADRATIC EQUATIONS
2008-11-13 6:11 am
1)If the speed of a car increased by 15km/h in a journey of 240km, 80 minutes would have been saved. Find the original speed of the car.
回答 (1)
2008-11-13 7:29 am
✔ 最佳答案
Let original speed = x.Time taken for a distance of 240km = 240/x.
New speed = x + 15
Time taken with new speed = 240/(x + 15).
Time saved = 80/60 = 4/3 hour. Therefore,
240/x - 240/(x +15) = 4/3
60/x - 60/(x+15) = 1/3
180(x+15) - 180x = x(x + 15)
2700 = x^2 + 15x
x^2 + 15x - 2700 = 0
(x-45)(x + 60) = 0
x = -60 (rej.) or 45.
so original speed = 45km/h.
收錄日期: 2021-04-25 22:37:46
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20081112000051KK01930