Help schoolmasters, teachers, this question is too small to understand. It is clearly written in the math book. A means positive opening and A means negative opening. Why are you here?
Because the quadratic term coefficient of quadratic function is 1, which is greater than 0, the quadratic function always opens upward. When a is positive, the linear function passes through the first, second and third quadrants, and the quadratic function has the minimum value, which is positive. When a is negative, the linear function passes through the first, second and fourth quadrants, and the quadratic function has the minimum value, which is negative.