(1)x^2+y^2=(x+y)^2-2xy=49-24=25(2)(x-y)^2=x^2+y^2-2xy=25-24=1 所以x-y=1
(x+y)^2=7^2 , x^2+2xy+y^2=7^2 , x^2+y^2=25
(x y)^2-2xy. 根号下[(x y)^2-4xy]
把X=7-Y代入XY=12
