第1个回答 2009-08-11
设
x+y=a
x-y=b 所以:
x=(a+b)/2
y=(a-b)/2
f(x+y,x-y)=xy+y*y就变成f(a,b)=(a+b)/2*(a-b)/2+(a-b)(a-b)/4
f(a,b)=(a*a-b*b+a*a-2ab+b*b)/4
f(a,b)=(a*a-a*b)/2
也就是f(x,y)=(x*x-x*y)/2
第2个回答 2009-08-11
则f(x+y,x-y)=xy+y*y 等于
fx+fy, f x-fy = xy+y*y
f(x,y)=xy+y*y - fx+fy
f(x,y)=y(x+y)-f(x-y)
所以f(x+y,x-y)=xy+y*y,则f(x,y)=_y(x+y)-f(x-y)__