For linear functions, the minimum value in a given interval must be obtained at one of the two endpoints. If the coefficient of x is unchanged, only the left end point is calculated; If the coefficient of x is always negative, calculate the right end point. But the coefficient x (m? -1) is indefinite, which may be positive, negative or 0, and the position of the minimum value is different according to different situations, so two endpoints should be calculated.

Or to m? Discussion on the classification of-1 symbol.