当x=0时y=0
当x>0时y=3x/(x*x+4)=3/[x+(4/x)]
由基本不等式,a>0,b>0时a+b>=2√[a*b],我就不证明了
y=3/[x+(4/x)]<=3/4
当x<0时
y=-3/[-x+(4/-x)]>=-3/4
综上所述,y属于[-3/4,3/4]