2x - 3y = a+1x + 2y = a2x - 3y = a+12x + 4y = 2a下面式减上面式子 消 x7y = a -1y = (a-1)/7x = a - 2y = a - 2(a-1)/7 = (5a +2)/7x>0y>05a + 2>0a-1>0a > -2/5a > 1所以 要使 x y 均为正数,则 a > 1...