y=5x-1/x+2
=(5x+10-11)/(x+2)
=(5x+10)/(x+2)-11/(x+2)
=5-11/(x+2)
因为 x+2≠0
所以 -11/(x+2)≠0
所以 5 -11/(x+2)≠5
值域为{y|y≠5}