3/(2+x)+3/(2+y)=1 ,x>1,y>1
通分得：3(4+x+y)=(x+2)(y+2)
xy-x-y-12=0
y=(x+12)/(x-1)
x+3y=x+3(x+12)/(x-1) = [x(x-1)+3(x+12)]/(x-1)
= (x²+2x+36)/(x-1)
= (x²-2x+1+4x-4+39)/(x-1)
= (x-1) + 39/(x-1) + 4
≥ 2√39 + 4