你好！设 a = x+y，b = 2x - y解得 x = (a+b)/3，y = (2a-b)/3z = x + 3y = (a+b)/3 + 2a - b = 7/3 a - 2/3 b1 ≤ a ≤ 3 ，7/3 ≤ 7/3 a ≤ 72 ≤ b ≤ 4，- 8/3 ≤ - 2/3 b ≤ - 4/3两式相加， - 1/3 ≤ 7/3 a - 2/3 b ≤ 17/3答案有误或者你打错题目
