x>1,y=3(x-1)+4/(x-1)+5,根据均值不等式，3（x-1)+4/(x-1)>=2√[3(x-1)*4/(x-1)=4√3,y(min)=4√3+5.
y=5+3(x-1)+4/(x-1)≤5+2√(3(x-1)*4/(x-1))=5+4√3当且仅当x=1+2√3/3
