Question

设函数∮(x,y)=xy,则∮(x+y,xy)=什么?_?

Answer 1

∮(x,y)=xy
变量替换，
∮(X,Y)=XY
x+y相当于上式的X，xy相当于Y，则
∮(x+y,xy)=(x+y)*xy=x²y+xy²

Answer 2

f(x,y)=xy/(x^2+y^2)

则f(y/x,1)
=(y/x)*1/[(y/x)^2+1^2]
=y/[x*(y^2/x^2+1)]
=y/(y^2/x+x)
=xy/(x^2+y^2)