假设a=1/xx趋于0则a趋于无穷所以就是求lim(a趋于无穷)sina/a只有a趋于0时sina/a的极限才等于1a趋于无穷则极限显然为0
|sin(1/x) |≤1, lim(x->0) x =0=> lim(x->0) xsin(1/x) =0
