When the coefficients of the first term and the second term are greater than 0, the function value can be obtained from the maximum and minimum values of the variables according to the range of the independent variables, and the sizes can be compared.

When the quadratic term and the coefficient of quadratic term are less than 0, the symmetry axis is within the range of independent variables. When the independent variable takes b/-2a, the function value is the largest, and the symmetry axis is within the range of independent variables, so that the function value can be obtained and compared.