按定义做.
lim(x-->0)|(f(x)-f(0))/x - 0|
=lim(x-->0)|x^2sin(1/x)|/|x|
=lim(x-->0) |xsin(1/x)|
因为 |sin(1/x)|0) |xsin(1/x)| =0
即 y'(0)=0