maths.F.2(before 10:00 tonight)

1.Let his sister ages y...then tom ages 3y
the difference of their ages is 36..
3y-y=36
2y=36
y=18
his sister ages 18...then tom ages 54(呢題有d怪..maybe我理解錯個題目)
2.if both the numerator and denominator of a fraction increase by 1, the value of the fraction becomes 1/2. if each of them decrease by 1, the value of the fraction becomes 1/4. find the fraction.
let the numerator be y.. denominator be z
(y+1)/(z+1)=1/2...(1)
(y-1)/(z-1)=1/4....(2)
In (1)
2(y+1)=z+1
2y+2=z+1
z=2y+1..(3)
Sub (3) into (2)
(y-1)/[(2y+1)-1)=1/4
4(y-1)=2y
4y-2y=4
y=2..(4)
Sub (4) into (3)
z=2x2+1
z=5
so the fraction is 2/5

2007-12-17 22:35:04 補充：
第一條一定不是9和27..因為 difference of their ages 不是36

1. Let Tom's sister be x years old, then Tom will be 3x years old,
so 3x - x = 36 gives x = 18 and 3x = 54

Therefore Tom is 54 and his sister is 18.

2. Let x be the numerator and y be the denominator,
Thus (x+1)(y+1) =1/2 gives 2(x+1) = y+1 or y = 2x + 1
and (x-1)/(y-1) = 1/4 gives 4(x-1) = y - 1 or y= 4x - 3

so y = 2x + 1 = 4x - 3 gives 2x = 4 or x = 2
so y = 2x +1 =2(2) + 1 = 5

so the fraction is 2/5

Let x be tom's sister ages
3x-x=36
2x=18
x=9
so that Tom's sister age is 9
tom is 27

chunwai320
但散

Y ???

3y-y =36

2y=18 (為什麼是 18)

2y=36 ????

y=9

2007-12-17 22:34:07 補充：
y=18

1.. let T be Tom 's sister's age
3T-T=36
2T=36
T=18
Tom is 54 and his sister is 18#

第二題我記得我做過
但我唔記得左點solve/___\

1. Let Tom's age be x, his sister's age be y,
x=3y ---(1)
x-y=36 ---(2)
Put (1) into (2),
3y-y =36
2y=18
y=9 ---(3)
Put (3) into (1),
x=27
Ages of tom is 27
Ages of his sister is 9

2. Let numerator and denominator of a fraction be (x/y),
(x+1)/(y+1) = 1/2
2x+2=y+1
2x-y+1=0 ---(1)
(x-1)/(y-1) = 1/4
4x-4=y-1
4x-y-3=0 ---(2)
(2)-(1):
2x-4=0
x=2 ---(3)
Put (3) into (1):
2(2)-y+1=0
y=5
Therefore, the fraction is (2/5)

1.
Let x be tom's sister ages
3x-x=36
2x=18
x=9
so that Tom's sister age is 9
tom is 27
唔知幫唔幫到你,第2條好難解= =