由题意得y不等于3
若y=3,则3x+3-3x=0,这显然是不成立的,故y不等3
所以,x=y/(y-3)
A= x+3y=y/(y-3) +3y
通分得 A=(3y^2 -8y)/(y-3)=f(y)
求导:f'(y)=[3(y^2-6y+8)]/(y-3)^2
令f'(y)=0 y=2 或 y=4
y=2时,x=-2
y=4时,x=4
故(x+3y)max=4+3*4=16