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1) A man drives 80 minutes for 60km. Find the speed on his return journey if the average speed for the whole journey is 60km/hr.
Let x is the speed of the retrun journey:
答覆: 120/(80+3600/x)=1 , x = 90km/hr.
q.1) 3600 這數字是什麼計來的及代表什麼?
q.2) 1 這數是什麼計來的及代表什麼?
2) 3Y-2X=Y+3X, find X^2 : Y^2
答覆:
5X = 2Y
X/Y = 2/5
so, X^2 : Y^2 = 4 : 25
q.1) 為什麼不是 X^2 : Y^2 = 25 : 4 ?
有數學題不明白原因
2012-03-14 9:57 pm
回答 (3)
2012-03-15 4:27 am
✔ 最佳答案
1) Let the speed of the return journey be x km/hr.speed=distance / time
time=distance / speed
The time required in the return journey is:
(60 km)/(x km/hr)=60/x hr=60X60/x min=3600/x min
Total distance traveled is: 2X60 km=120 km
Total time required is: 80 min+3600/x min=(80+3600/x) min
The average speed for the whole journey is:
(120 km)/[(80+3600/x) min]=120/(80+3600/x) km/min
The average speed for the whole journey is:
60 km/hr=60/60 km/min=1 km/min
120/(80+3600/x)=1
120=80+3600/x
3600/x=40
40x=3600
x=90
答案: The speed of the return journey is 90 km/hr.
q.1) 3600 這數字是什麼計來的及代表什麼?
1小時有60分鐘
60/x小時=60/x X 60分鐘=60X60/x分鐘=3600/x分鐘
q.2) 1 這數是什麼計來的及代表什麼?
平均速度=60 km/hr=60 km/60 min=1 km/min
2) 3Y-2X=Y+3X
3Y-Y=3X+2X
5X=2Y
X/Y=2/5
(X/Y)^2=(2/5)^2
(X^2)/(Y^2)=(2^2)/(5^2)
(X^2)/(Y^2)=4/25
so, X^2 : Y^2 = 4 : 25
q.1) 為什麼不是 X^2 : Y^2 = 25 : 4 ?
若 a / b = 1 / 2
則 a : b = 1 : 2
2012-03-17 6:18 am
You can just take the '/' as ':'
2012-03-15 9:36 am
Another way:
Let t = time for return journey
60 km/h = total distance/total time taken
120 km/[(80/60) + t ] = 60 km/h
120 km =60[(80/60) + t]
2 =(80/60) + t
t = 2h -80/60 h = (2 – 4/3)h = 2/3 h
Speed on return journey = 60 km/(2/3 h)= 90 km/h
Let t = time for return journey
60 km/h = total distance/total time taken
120 km/[(80/60) + t ] = 60 km/h
120 km =60[(80/60) + t]
2 =(80/60) + t
t = 2h -80/60 h = (2 – 4/3)h = 2/3 h
Speed on return journey = 60 km/(2/3 h)= 90 km/h
收錄日期: 2021-04-29 17:05:49
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20120314000051KK00296