問數學題～

#65
according to the graph , we observe that the common point of L1 and L2 is (-4,-7),so the answer is B
#66
we sub. x=-y into 4x+y-15=0
we obtain x=5,y=-5
the answer is A
#67
we sub.y=2x+3 into y+4x-23=0
x=10/3
so the answer is B
#68
due to the (1,1) satisfies the two equations
therefore,we sub x=1,y=1 into the two equations
a+1=3 >a=2
b+1=5 > b=4
so the answer is C
#69
we sub. y=8-3x into 2x-1=y+1
x=2
so the answer is A
#70
we have two equations
y+3x-10=0---------1
y=2x------2
sub. 2 into 1
x=2,y=4
therefore,x-y=-2
the answer is D
#71
we assume x=a,y=2a
x+y=2
3a=2
a=2/3
x=2/3,y=4/3
the answer is D
#72
for a rectangle ,
we know AD=BC,AB=CD
2x=2y-x
x=2/3y--------------1
5y-6x+8=2x+y------------2
sub.1 into 2
y=6,x=4
the answer is B

65. B
66. A
67. B
68. C
69. A
70. D
71. C
72. B