急！！！數學use of formulae問題x1！！！

2008-07-25 10:26 pm

1.Town A and B are d km apart. A boy used x hours to run from A to B at a
speed of u km/h and then uses y hours to walk back to A at v km/h.
1.1:Find y in terms of u and d.
1.2:Find the average speed for the whole journey of the boy in terms of u and v.
請列式～～～ｔｈｘ！！

更新1:

eelyw: 咁1.1題既式係咪姐係好似你果條: u=d/x................(1) v=d/y................(2) 咁= =但我都係唔係幾明

更新2:

sor,打少左1題: h既2次方+k既2次方 ------------------------- T=2兀乘 g既平方根

更新3:

sor果題打得唔好,再打多次= = 姐係T=2兀乘 g既平方根分之h既2次方+k既2次方 thx~~~

回答 (2)

wai hei

2008-07-25 11:10 pm

✔ 最佳答案

1.1 ux=d and vy=d
ie ux=vy
y=ux/v
but i have no idea for the next step

1.2 the average speed= 2d/(x+y)
= 2d/(d/u+d/v)
= 2d/ [(dv+du)/vu]
= 2d * vu/[d(v+u)]
= 2vu/(v+u)

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用戶10012

2008-07-25 11:03 pm

u=d/x................(1)
v=d/y................(2)
From (2) y = d/v.
Total time taken = x + y.
Total distance travelled = 2d.
Therefore, average speed = 2d/(x+y) = 2d/(d/u + d/v) = 2/(1/u + 1/v)
= 2uv/(u+v).

2008-07-25 16:12:55 補充：
Cannot express y in terms of u and d because the 2 equations has no relationship on u. You can only express y in terms of v and d.

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收錄日期: 2021-04-25 22:36:47
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