math f.1

In 3 consecutive number , the smallest number is 19/20 of the largest one .find the middle number.
在三個連逐數中，最細的數字是最大的數字的１９／２０倍
找出中間的數。
Let x-1 , x , x+1 be the 3 consecutive number <--盡量只用一個未知數,這
會方便運算
since the smallest number is 19/20 of the largest one ,
so
x-1 = (19/20) (x+1)
20x-20 = 19x + 19
x = 39
since x is the middle number ,so the answer is 39.

let 3 consecutive numbers are x-1 , x , x+1

the smallest number is 19/20 of the largest one

=> x-1=(19/20)(x+1)
=>20x-20=19+19x
=>x=39//

middle number.=39//

Let x and y be the smaller number and the largest number

1. x = y*19/20 <-------如題
2. y-x=2 <------x 和y的差
x = y-2

Put 2 into 1

x = y*19/20
y-2 = y*19/20
20/20*y-2 = y*19/20<---------20/20*y =1y = y
20/20*y-y*19/20 = 2
1/20 y = 2
y = 40

therefore x =38 and the middle number is 39

2008-05-29 17:37:41 補充：
這個是二元一次方的方法
中二會學, 學定也不妨