bài 2
x/9 = y/4 <=> 4x = 9y <=> x = 9/4y
a) x + y = 65 
<=> 9/4y + y = 65 <=> 13/4y = 65 <=> y = 20
=> x = 9/4.20 = 45
b) x-y = 65
<=> 9/4y - y = 65
<=> 5/4y = 65
<=> y = 52
=> x = 9/4.52 = 117
c) 3x+2y = 105
<=> 3.9/4y + 2y = 105
<=> 27/4y + 2y = 105
<=> 35/4y = 105
<=> y = 12
=> x = 9/4.12 = 27