1/ x + 1/y = 1/12
8/x + 15/y = 1 
Đăt a = 1/x
b = 1/y
=> a+ b = 1/12  => 8a + 8b = 2/3 (1)
8a + 15b = 1  (2)
=>  (1) - (2) => -7b = -1/3 => b = 1/21 = > y = 21 
=> a = 1/12 - 1/21 = 1/28  => a = 28

b) 2/x + 2y +  1/y+2x = 3
  4/ x + 2y - 1/ y+2x  = 1
=> 2a + b = 3
4a - b = 1 
=> 6a  = 4 => a = 2/3  => x+2y = 3/2 
b = 4 . 2/3 - 1 = 5/3  => y  + 2x = 3/5  
=> x= -1/10
y = 4/5