(x-y)²=x²+y²-2xy=4-2xy=1 2xy=3 (x+y)^2=x²+y²+2xy=4+3=7x^2y^2=(xy)²=4分之9 追答 (x-y)²=x²+y²-2xy=4-2xy=1 2xy=3 (x+y)^2=x²+y²+2xy=4+3=7x^2y^2=(xy)²=4分之9