中二數學 急急急~有15點

1.
let x be Alex's age and y be his daughter's age
x = 3y .......(1)
x - y = 36....(2)
Sub (1) into (2),
3y - y = 36
2y = 36
y = 36/2
y = 18
Sub y=18 into (1),
x = 3*18
x = 54
therefore the respective ages of Alex and his daughter is 54 and 18.


2.
let A be Christina's age and B be her son's age
B = A - 30 .........(1)
A+3 = 3*(B+3)....(2)
Sub (1) into (2),
A+3 = 3*[(A-30)+3]
A+3 = 3A-90+9
3+90-9 = 3A-A
84 = 2A
A=84/2
A=42
therefore, the current age of Christina is 42


3.
let x be the length of the rectangle and y be the width.
x = 5y ........... . (1)
(x+y)*2 = 48......(2)
Sub (1) into (2),
(5y+y)*2 = 48
6y = 48/2
6y = 24
y = 24/6
y = 4
Sub y=4 into (1),
x=5*4
x=20
Therefore, the length and width of the rectangle are 20cm and 4cm respectively.


4.
let x be the length of AB and y be the length of AC
x = 12+y.............(1)
x+x+y = 96.........(2)
Sub (1) into (2),
2x+y=96
2(12+y)+y = 96
24+2y+y = 96
3y =96-24
3y = 72
y = 72/3
y = 24
therefore, the length of AC is 24cm