c = -2c/a = x1x2 > 0 ===> a < 0-b/a = x1 + x2 ===> b = -a(x1 + x2)3a + b = 3a - a(x1 + x2)= a(3 - x1 - x2)x1 < 2,x2 < 2 ===> x1 + x2 < 3 ===> 3 - x1 - x2 > 0 ===> a(3 - x1 - x2) > 0选b