聯立方程(20點)

27.) 2y+1=x-1-------(1)
2x-y-1=x+2y ---------(2)
From(2), x=3y+1----------(3)
Sub(3)into(1), 2y+1=3y+1-1
y=1
Sub y=1 into(3)
x=3+1
=4
Hence, Area of rectangle = (4-1)(4+2*1)
= 3*6
= 18cm^2
28.)
Let the number in ten digit be x, and the one in integer digit bey,
10x+y=4(x+y)
6x=3y
2x=y---------(1)
10y+x+9=2(10x+y)
8y+9=19x -----------(2)
Sub(1) into(2), 8(2x)+9=19x
16x+9=19x
3x=9
x=3
Sub x=3 into(1),
y=2*3
=6
Hence, the number is 36

由於長乘闊等於面積，所以

28. 2y+1=x-1(1)
2x-y-1=x+2y(2)
from(1) x=2y+2
from(2) x=3y+1
2y+2=3y+1
y=1
x=4
ans:(2*1+1)(4+2*1)=18
29. 設x=十位 y=個位
10x+y=4(x+y)(1)
10y+x+9=20x+2y(2)
from(2)
19x-8y=9(3)
from(1)
10x+y=4x+4y
6x=3y
x=y/2(4)
把(4)套入(3)
(19/2)y-8y=9
(3/2)y=9
y=6
x=3
ans:36