y=ax^2+bx+c。 (a is not equal to 0)

When a>0, the opening of the quadratic function is upward. At this time, depending on the minimum value of the function, that is, the lowest point of the parabola, the minimum value is obtained at the symmetry axis.

Of course, it is also useful to look at the discriminant Delta. We know that when d is 0, the image has two intersections with the X axis. At this time, the part of the value of y between these two points is less than 0, and the part outside these two points is greater than 0. When D=0, there is only one intersection point between the image and the X axis, and the minimum value of the function is 0. When d "0", the images do not intersect with the X axis, that is, all images are above the X axis, and Y "0".

The situation of a 0 is similar.