问题 解答题

已知:x+y="3" ,xy="1" ,试求:

(1)x2 + y2的值;              

(2)(x-y)2的值.

答案

7,5

(1)根据(x+y)2=x2+2xy+y2,变形即可;

(2)根据(x-y)2=(x+y)2-4xy,整体代入计算

解:(1)x2+y2

=(x+y)2-2xy

=9-2

=7;

(2)(x-y)2

=(x+y)2-4xy

=9-4

=5.

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