已知x+y=2,xy=-3,则x-y的值为

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已知x+y=2,xy=-3,则x-y的值为已知x+y=2,xy=-3,则x-y的值为已知x+y=2,xy=-3,则x-y的值为(x-y)^2=(x+y)^2-4xy=2^2+12=16所以：x-y=4

已知x+y=2,xy=-3,则x-y的值为
已知x+y=2,xy=-3,则x-y的值为

已知x+y=2,xy=-3,则x-y的值为
(x-y)^2=(x+y)^2-4xy=2^2+12=16
所以：x-y=4或x-y=-4

x=2-y带入xy=-3中
（2-y）y=-3；
y^2-2y-3=0;
y=-1或y=3,所以：
x=3或x=-1，则：
x-y=4或-4

由题意可知，y=2-x;
x(2-x)=-3
化简可得方程：x*x-2x-3=0
（x-3）（x+1）=0
x=3或x=-1
y=-1或y=3
所以x-y为-4或4

(x-y)(x-y)=(x+y)(x+y)-4xy = 2x2+12=16
x-y=4或-4
当x-y=4时， x=3, y=-1
当x-y=-4时， x=-1, y=3

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