(2/3)x+(3/4)y=1/2(1)x12得8x+9y=6 (3) (4/5)x+(5/6)y=7/15(2)x30 得24x25y=14 (4) 把(3)x3得 24x+27y=18 (5)把(4)-(5) 得 -2y=-4 即y=2把y=2代入(3)算出x=-(3/2)...