y=(3+x)/(4-x)=-(x+3)/(x-4)
=-(x-4+7)/(x-4)
=-[(x-4)/(x-4)+7/(x-4)]
=-[1+7/(x-4)]
=-1-7/(x-4)
因为7/(x-4)≠0
所以y≠-1-0=-1
所以值域(-∞,-1)∪(-1,+∞)
2x^2-4x+3=2(x-1)^2+1>=1
所以0