(x-y)²=x²-2xy+y²=(x+y)²-4xy当x+y=7，xy=12时，代入得7²-4*12=1
(x-y)²=x²-2xy+y²=(x+y)²-4xy=49-48=1
（x-y)^2=x^2-2xy+y^2=x^2+2xy+y^2-4xy=(x+y)^2-4xy=7^2-4*12=1
1
