lim(x->0) f(x)=lim(x->0) x.arctan(1/x)=0 = f(0)=> x= 0 , f(x) 连续f'(0) = lim(h->0) [f(h) - f(0)] /h= lim(h->0) [ harctan( 1/h) ] /h= lim(h->0) arctan( 1/h) =π/2
