f(x+y,x-y)=2xy(x-y),求f(x,y)

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f(x+y,x-y)=2xy(x-y),求f(x,y)
f(x+y,x-y)=2xy(x-y),求f(x,y)

f(x+y,x-y)=2xy(x-y),求f(x,y)
f(x+y,x-y)=(1/2)[(x+y)+(x-y)][(x+y)-(x-y)](x-y)
所以f(x,y)=(1/2)(x+y)(x-y)·y=(1/2)(x²y-y³)

1.换元,令x+y=m & x-y=n
则x=(m+n)/2,y=(m-n)/2
2.回代,得f(m,n)=2((m+n)/2)((m-n)/2)((m+n)/2-(m-n)/2)=n(m^2-n^2)/2
3.m,n是符号变量 可以改写成x,y 则,f(x,y)=y(x^2-y^2)/2