题目设x,y,z为正数,求xy+2yz/x2+y2+z2的最大值.文[1]中潘继军老师巧用均值不等式解答了上面这个式子的最大值,并给出了3个变式和两个引理,读后很受启发.从教学的角度来讲,笔者认为文[1]中的变式还远远不够,文[1]的三个变式属于同一类型,解答方法有一定的局限性.本文用参数均值不等式的方法来解答这个题目的各种变 The topic set x, y, z is positive, and find the maximum of xy +2 yz / x2 + y2 + z2. [1] Pan Jijun teacher clever use of means of inequality to answer the above formula of the maximum, and gives Three variants and two lemmas, read very inspired.From the teaching point of view, I believe that the text [1] in the variant is not enough, the text [1] of the three variants belong to the same type , There are some limitations of the answer method.In this paper, the method of parameter mean inequality to answer the various topics of this variable