x趋于无穷大(x^3*sin(1/x))/(x^2+1)=
x趋于无穷大(sin(1/x))/1/x+1/x^3)=
t趋于0 (sin(t))/(t+t^3)=t趋于0 sin(t))/t=1
原极限为 无穷/无穷 型的