minimize -2x1+3x2 maximize 2x1 3x2 subject to subject to x1+ X2 =7 x1+ X2 =7 x1- 2x2 ≤ 4 x1-2x2 4 xI 0. X1 ≥0 maximize 2x1-3x2+3x2 subject to x1+ x =7 ←☐令X2=X2'-X2 x1-2x 2x2 ≤4 x1.x2.x2 0. 很“无厘 头”？令x2=x2 ’-x2 ” 很“无厘 头”？