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遇到与二次根式有关的求值问题,若能根据其结构特征,灵活运用各种代换策略,则能使运算化难为易,迅速获解．一、整体代换例1已知x=(3~(1/2)-2~(1/2))/(3~(1/2)+2~(1/2)),y=(3~(1/2)+2~(1/2))/(3~(1/2)-2~(1/2)),求代数式3x~2-5xy+3y~2的值．解∵x=(3~(1/2)-2~(1/2))/(3~(1/2)+2~(1/2))=(3~(1/2)-2~(1/2))~2=5-26~(1/2)．y=(3~(1/2)+2~(1/2))/(3~(1/2