1, discriminant method is an important bridge between equality and inequality. If it can be skillfully used in the process of solving the maximum value of multivariate function, it can give people a simple and refreshing feeling. The core of applying discriminant lies in whether quadratic equation or quadratic function can be constructed reasonably and whether equal sign can be taken should be paid attention to.

2. Formula method is mostly used for quadratic function. Through variable substitution, it can be transformed into a quadratic function form about t(x). The function can be formulated as f(x)=a[t(x)m]2+n, and then its maximum value can be determined according to the properties of quadratic function. The key to solve this kind of problem lies in transforming quadratic function into vertex by matching method, and at the same time, the abscissa of vertex should be considered.

3. In order to get the maximum value of average inequality, three necessary conditions must be met: positive, definite and three-phase. Therefore, when some of these conditions are not satisfied, we should consider appropriate identical deformation to satisfy these conditions, and determine the conditions for the maximum product, the definite product and the minimum product, especially the equal sign.