已知f(xy,x-y)=x2+y2,

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已知f(xy,x-y)=x2+y2,已知f(xy,x-y)=x2+y2,已知f(xy,x-y)=x2+y2,设s=xy,t=x-y,则x^2+y^2=(x-y)^2+2xy=t^2+2s,所以f(s,

已知f(xy,x-y)=x2+y2,
已知f(xy,x-y)=x2+y2,

已知f(xy,x-y)=x2+y2,
设 s=xy,t=x-y ,
则 x^2+y^2=(x-y)^2+2xy = t^2+2s ,
所以 f(s,t) = t^2+2s ,也即 f(x,y) =y^2+2x ,
所求和为 2+2y .选 A

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