math problems

2007-12-04 5:57 am

1. Make d the subject of the formula S = n/2 〔2a + (n - 1)d 〕.

2.The price of a pack of balloons and a pack of paper cups are $3 and $5 respectively. In a party, $61 is spent on buying balloons and paper cups. If the total number of packs of balloons and paper cups are 15, find the number of packs of balloons bought.

回答 (1)

大白Queen

2007-12-04 6:20 am

✔ 最佳答案

1)
S = n/2 〔2a + (n - 1)d 〕
2S/n = 2a + (n - 1)d
2S/n -2a = (n-1)d
(2S-2an) /n = (n-1)d
d = (2S-2an) /(n^2-n)

2)
Let the number of packs of balloons are x and the number of packs of paper cups are y

3x+5y = 61 ..............(1)
x+y = 15 ...............(2)

From (2)
y = 15-x .............(3)

Sub (3) into (1)
3x+5y = 61
3x + 5(15-x) = 61
3x +75 -5x = 61
-2x = -14
x = 7

There are 7 packs of balloons

另外 ,Sub x = 7 into (2)
x + y = 15
7 + y = 15
y = 8

There are 8 packs of paper cups

參考: 經驗所得

👍 0 👎 0