已知x+y=4,x²-y²=20,求x-y

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x²-y²=20=(x-y)(x+y)=4(x-y)
x-y=5

x²-y²=(x+y)(x-y)=4(x-y)=20
x-y=5

x²-y²=20
所以（x+y)(x-y)=20
所以x-y=5