已知x^2+xy=12,xy+y^2=15,求(x+y)^2及（x+y)（x-y)的值

x^2+xy=12,
xy+y^2=15,
x^2+xy+xy+y^2=12+15
(x+y)^2=27
x^2+xy=12,
xy+y^2=15
x^2-y^2=-3
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