一條數題目~唔識做..救命...

2007-01-03 12:09 am
The charge of the tap water supply $W is calculated based on the consumption (c m^3 ) in four months.The first 12m^3 of tap water consumed is free of charge, and the charge for every m^3 consumed afterwards is $4.5.

(a) Write down a formula calculating the value of W for c > 12.
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(b) If John consumed 25m^3 of water in the past four months, how much should he pay ?

**註:上面的>是 greater than or equal to **
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回答 (2)

2007-01-10 8:02 am
✔ 最佳答案
(a) For c ≥ 12
W = 4.5(c - 12)

解釋:
每使用1m^3的水都要俾$4.5,所在計算W時要將使用量乘4.5
原本W應該等於4.5 x c,即係W = 4.5c
但係佢話到頭12m^3既水都係免費既
所以,我地就要在使用量c上減去12,即(c - 12)
因此W = 4.5(c - 12)

要注意一點係W = 4.5(c - 12)中既c係唔可以細過12
因為會出負數,咁就唔係交水費,而係收錢
所以題目就列明c要大過或者等於12

(b) W
= 4.5(25 - 12)
= 4.5(13)
= 58.5
∴ He should pay $58.5

解釋:
其實只係照代公式
因為John既使用量係25m^3,所以乎合(a) part既條件(即係c ≥ 12)
如果唔乎合(a) part既條件,就唔可以照代公式
例如佢問你
If John consumed 10m^3 of water in the past four months, how much should he pay ?
咁你就唔可以代公式,因為c = 10 < 12
而個答案當然就係0啦

這題亦都有一個地方要注意的
如果將計算步驟寫成
W
= $4.5(25 - 12)
= $4.5(13)
= $58.5
係會被扣分
因為題目上已為W標上單位$
如果代W = $25入去就會出現$($25) = ($^2) 25
錢係無二次既,所以錯

希望咁詳盡既解釋可以幫到你
2007-01-03 12:18 am
(a) For c is greater than or equal to 12,
W = 4.5(c-12)
= 4.5c - 54


(b) He should pay:
put c=25
W = 4.5(25) - 54
= 112.5 - 54
= 58.5
So, he should pay $58.5

Hopes can help u
參考: myself


收錄日期: 2021-04-12 21:27:06
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