Question

f(x,y)=x+2y,求f(xy,f(x,y))

Answer 1

xy+2(x+2y)

=xy+2x+4y

令x+y=m y/x=n

联立解得

x=m/(n+1) y=mn/(n+1)

所以f(m,n)=x^2-y^2

=m^2(1-n^2)/(n+1)^2=m^2(1-n)/(1+n)

即f（x,y)=x^2(1-y)/(1+y)