设y=(5x+3)/(3x^2+5)
lim y=0
lim sin(4/x)=0
lim sin(4/x)/y=lim cos(4/x)(-4/x^2)/y'
y'=-(15x^2+18x-25)/(3x^2+5)^2
lim sin(4/x)/y=lim cos(4/x)(-4/x^2)/y'=lim cos(4/x)/y2
其中y2=x^2*(15x^2+18x-25)/(3x^2+5)^2
lim cos(4/x)=1
lim y2=15/9=5/3
故
lim(3x^2+5)/(5x+3)*sin4/x=1/(5/3)=3/5