y = x-2x-3 = x-2x+1 -1 -3 = (x-2x+1)-4 = (x-1)-4 x是任意实数,即(﹣无穷,+无穷) 因为(x-1)≥0 所以y = (x-1)-4 ≥ 0-4 = -4 所以y ≥ -4