y=(3x+1)/(x+2)
=(3x+6-5)/(x+2)
=3-5/(x+2)
(1)
当x≠-2 时 y∈R且y≠3
(2)
当x>=0
x+2>=2
0=-5/2
3-5/(x+2)>=3-5/2=1/2
所以 y>=1/2