(* * * * * * Therefore, the symmetry axis x=-2/a of its image must be between x=4 and x=6, with a bias of x=5, that is,-1/2.

This sentence means something like this: can you y=n? The quadratic form of +an(n is a real constant) is first regarded as y=x? +ax=(x+a/2)？ -a？ /4 the solution when x is a positive integer in this parabola with an upward opening. (Note that a quadratic graph is a series of points on a curve, not the whole curve.)

Then if and only if n=5, y has the minimum value in the topic, that is to say, when x=5, the point corresponding to y value is the minimum value. You can draw a picture to see if the axis of symmetry is between x=4 and x=6, and it is possible to ensure that x=5 has a minimum value (note that it is possible now).

In the analysis, the deviation of x=5 is to ensure that the series of discrete points of quadratic form is the smallest, because if x=5 is not deviated, the minimum value may be taken at the point of x=4 or x=6. Think again.

Because X is a discrete point when it is an integer, and the symmetry axis moves within the range of x=5 and does not exceed 1/2, it can be completely guaranteed that the integer point value of the quadratic formula is the smallest when x=5, not when x=4 or x=6. That's why-1/2

It should be noted that the quadratic formula has a minimum value when x=5, which is not necessarily the minimum value of the whole parabola. The minimum value of the whole parabola must be the y value on the axis of symmetry of x=-a/2, but x=-a/2 is not necessarily an integer.