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合理构造两数和与积求解数学问题，是一种非常有效的手段．其独特功能在于充分运用一元二次方程根的判别式和求根公式变更命题，从而使问题获得简解1用于求值例1已知：为整数），那么，的值是（）．（A）1991-1（B）－1991－1（C）（－1）n1991（D）（－1）n1991－1（1991年全
It is a very effective method to construct the two-number sum sum product to solve mathematical problems. Its unique function is to make full use of the discriminant of the root of the quadratic equation and to find the root formula to change the proposition, so that the problem can be solved briefly. 1 is used for evaluation. Example 1 is known as an integer. Then, the value is (). (A) 1991-1(B)-1991-1(C)(-1)n1991(D)(-1)n1991-1 (full year 1991)