lim x趋于无穷 sinx/x=0
lim x趋于无穷 x/sin1/x->无穷/0型还是无穷
lim x趋于无穷 xsin1/x=(sin1/x)/(1/x)=1
lim x趋于0 xsin1/x=0
lim x趋于0 1/xsinx=1
