数学问题中条件有明有暗,明者易于发现便于应用,暗者隐含于有关概念、知识的内涵之中,含而不露,极易忽视,稍不留心便导致解题出错.今举几例,告诫同学,解题时提高警惕,以防错解.例1 已知 x≥0,y≥0,且 x+2y=1,求2x+3y~2的最值.错解由 x+2y=1,得 x=1-2y.令 u=2x+3y~2,则u=2(1-2y)+3y~2=3(y-(2/3))~2+(2/3), The conditions of the mathematics problems are clear and dark, and the bright ones are easy to find and easy to apply. The dark ones are implicit in the connotation of the concepts and knowledge, but they are not exposed and easily overlooked. A little care is taken to cause problems in solving problems. For example, admonish classmates and raise vigilance when solving problems to prevent misunderstandings. Example 1 Given that x≥0, y≥0, and x+2y=1, find the best value of 2x+3y~2. The wrong solution is x+ 2y=1, get x=1-2y. Let u=2x+3y~2, then u=2(1-2y)+3y~2=3(y-(2/3))~2+(2/ 3),